feat(joinir): Phase 92 P0-3 - ConditionalStep → JoinIR Select generation

Complete implementation of P5b escape sequence handling pattern lowering:

Core changes:
- skeleton_types.rs: Add `cond` field to ConditionalStep for escape condition
- escape_pattern_recognizer.rs: Extract and return escape_cond AST
- pattern_recognizer.rs: Pass escape_cond to ConditionalStep
- canonicalizer.rs: Store escape_cond in ConditionalStep variant

JoinIR lowering:
- carrier_update_emitter.rs: Add emit_conditional_step_update() function
  - Lower condition AST via lower_condition_to_joinir
  - Compute both then/else branches (carrier + delta)
  - Emit JoinInst::Select for conditional carrier update
- loop_with_break_minimal.rs: Add skeleton parameter, detect ConditionalStep
- pattern2_with_break.rs: Pass skeleton to lowering function

Backward compatibility:
- skeleton=None preserves existing Pattern1-4 behavior
- fixtures.rs, tests.rs updated for new signature

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/escape_pattern_recognizer.rs

@@ -20,6 +20,9 @@ pub struct EscapeSkipPatternInfo {
     pub quote_char: char,
     pub escape_char: char,
     pub body_stmts: Vec<ASTNode>,
+    /// Phase 92 P0-3: The condition expression for conditional increment
+    /// e.g., `ch == '\\'` for escape sequence handling
+    pub escape_cond: Box<ASTNode>,
 }
 
 /// Detect escape sequence handling pattern in loop body
@@ -57,8 +60,9 @@ pub fn detect_escape_skip_pattern(body: &[ASTNode]) -> Option<EscapeSkipPatternI
     // Find escape check after break - scan for second "if" with increment
     let escape_idx = find_escape_in_if(body, break_idx)?;
 
-    // Extract counter_name, escape_delta, and normal_delta from the escape if statement
-    let (counter_name, escape_delta, normal_delta) = extract_delta_pair_from_if(body, escape_idx)?;
+    // Extract counter_name, escape_delta, normal_delta, and condition from the escape if statement
+    // Phase 92 P0-3: Now also extracts condition expression
+    let (counter_name, escape_delta, normal_delta, escape_cond) = extract_delta_pair_from_if(body, escape_idx)?;
 
     // Extract body statements before break check
     let body_stmts = body[..break_idx].to_vec();
@@ -70,6 +74,7 @@ pub fn detect_escape_skip_pattern(body: &[ASTNode]) -> Option<EscapeSkipPatternI
         quote_char: '"',  // Default for JSON/CSV (Phase 91 MVP)
         escape_char: '\\',  // Default for JSON/CSV (Phase 91 MVP)
         body_stmts,
+        escape_cond,  // Phase 92 P0-3: Condition for JoinIR Select
     })
 }
 
@@ -139,17 +144,20 @@ fn find_escape_in_if(body: &[ASTNode], after_idx: usize) -> Option<usize> {
     None
 }
 
-/// Extract both escape_delta and normal_delta from if statement
+/// Extract escape_delta, normal_delta, and condition from if statement
 ///
 /// Handles both:
 /// - if ch == escape_char { i = i + 2 } else { i = i + 1 }
 /// - if ch == escape_char { i = i + 2 } (followed by separate increment)
-fn extract_delta_pair_from_if(body: &[ASTNode], idx: usize) -> Option<(String, i64, i64)> {
+///
+/// Phase 92 P0-3: Now returns the condition expression for JoinIR Select generation
+fn extract_delta_pair_from_if(body: &[ASTNode], idx: usize) -> Option<(String, i64, i64, Box<ASTNode>)> {
     if idx >= body.len() {
         return None;
     }
 
     if let ASTNode::If {
+        condition,
         then_body,
         else_body,
         ..
@@ -204,7 +212,8 @@ fn extract_delta_pair_from_if(body: &[ASTNode], idx: usize) -> Option<(String, i
             found_delta?
         };
 
-        Some((counter_name, escape_delta, normal_delta))
+        // Phase 92 P0-3: Return condition along with deltas
+        Some((counter_name, escape_delta, normal_delta, condition.clone()))
     } else {
         None
     }

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -717,6 +717,7 @@ pub(crate) fn can_lower(builder: &MirBuilder, ctx: &super::router::LoopPatternCo
 ///
 /// Wrapper around cf_loop_pattern2_with_break to match router signature
 /// Phase 200-C: Pass fn_body to cf_loop_pattern2_with_break
+/// Phase 92 P0-3: Pass skeleton for ConditionalStep support
 pub(crate) fn lower(
     builder: &mut MirBuilder,
     ctx: &super::router::LoopPatternContext,
@@ -727,6 +728,7 @@ pub(crate) fn lower(
         ctx.func_name,
         ctx.debug,
         ctx.fn_body,
+        ctx.skeleton,  // Phase 92 P0-3: Pass skeleton for ConditionalStep
     )
 }
 
@@ -735,6 +737,7 @@ impl MirBuilder {
     ///
     /// **Refactored**: Now uses PatternPipelineContext for unified preprocessing
     /// **Phase 200-C**: Added fn_body parameter for capture analysis
+    /// **Phase 92 P0-3**: Added skeleton parameter for ConditionalStep support
     ///
     /// # Pipeline (Phase 179-B)
     /// 1. Build preprocessing context → PatternPipelineContext
@@ -754,10 +757,12 @@ impl MirBuilder {
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
         // Phase 200-C: Delegate to impl function with fn_body=None for backward compatibility
-        self.cf_loop_pattern2_with_break_impl(condition, _body, _func_name, debug, None)
+        // Phase 92 P0-3: skeleton=None for backward compatibility
+        self.cf_loop_pattern2_with_break_impl(condition, _body, _func_name, debug, None, None)
     }
 
     /// Phase 200-C: Pattern 2 implementation with optional fn_body for capture analysis
+    /// Phase 92 P0-3: Added skeleton parameter for ConditionalStep support
     fn cf_loop_pattern2_with_break_impl(
         &mut self,
         condition: &ASTNode,
@@ -765,6 +770,7 @@ impl MirBuilder {
         _func_name: &str,
         debug: bool,
         fn_body: Option<&[ASTNode]>,
+        skeleton: Option<&crate::mir::loop_canonicalizer::LoopSkeleton>,
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_break_minimal::lower_loop_with_break_minimal;
 
@@ -869,6 +875,7 @@ impl MirBuilder {
             analysis_body, // Phase 191/192: Pass normalized body AST for init lowering
             Some(&mut inputs.body_local_env), // Phase 191: Pass mutable body-local environment
             &mut inputs.join_value_space, // Phase 201: Unified ValueId allocation (Local region)
+            skeleton, // Phase 92 P0-3: Pass skeleton for ConditionalStep support
         ) {
             Ok((module, meta)) => (module, meta),
             Err(e) => {

src/mir/join_ir/lowering/carrier_update_emitter.rs

@@ -362,6 +362,101 @@ pub fn emit_carrier_update(
     }
 }
 
+// ============================================================================
+// Phase 92 P0-3: ConditionalStep Support
+// ============================================================================
+
+use crate::ast::ASTNode;
+use crate::mir::join_ir::lowering::condition_lowerer::lower_condition_to_joinir;
+use crate::mir::join_ir::VarId;
+use crate::mir::MirType;
+
+/// Emit JoinIR instructions for conditional step update (Phase 92 P0-3)
+///
+/// Handles the P5b escape sequence pattern where carrier update depends on a condition:
+/// ```text
+/// if escape_cond { carrier = carrier + then_delta }
+/// else { carrier = carrier + else_delta }
+/// ```
+///
+/// This generates:
+/// 1. Lower condition expression to get cond_id
+/// 2. Compute then_result = carrier + then_delta
+/// 3. Compute else_result = carrier + else_delta
+/// 4. JoinInst::Select { dst: carrier_new, cond: cond_id, then_val: then_result, else_val: else_result }
+///
+/// # Arguments
+///
+/// * `carrier` - Carrier variable information (name, ValueId)
+/// * `cond_ast` - AST node for the condition expression (e.g., `ch == '\\'`)
+/// * `then_delta` - Delta to add when condition is true
+/// * `else_delta` - Delta to add when condition is false
+/// * `alloc_value` - ValueId allocator closure
+/// * `env` - ConditionEnv for variable resolution
+/// * `instructions` - Output vector to append instructions to
+///
+/// # Returns
+///
+/// ValueId of the computed update result (the dst of Select)
+pub fn emit_conditional_step_update(
+    carrier: &CarrierVar,
+    cond_ast: &ASTNode,
+    then_delta: i64,
+    else_delta: i64,
+    alloc_value: &mut dyn FnMut() -> ValueId,
+    env: &ConditionEnv,
+    instructions: &mut Vec<JoinInst>,
+) -> Result<ValueId, String> {
+    // Step 1: Lower the condition expression
+    let (cond_id, cond_insts) = lower_condition_to_joinir(cond_ast, alloc_value, env)?;
+    instructions.extend(cond_insts);
+
+    // Step 2: Get carrier parameter ValueId from env
+    let carrier_param = env
+        .get(&carrier.name)
+        .ok_or_else(|| format!("Carrier '{}' not found in ConditionEnv", carrier.name))?;
+
+    // Step 3: Compute then_result = carrier + then_delta
+    let then_const_id = alloc_value();
+    instructions.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: then_const_id,
+        value: ConstValue::Integer(then_delta),
+    }));
+    let then_result = alloc_value();
+    instructions.push(JoinInst::Compute(MirLikeInst::BinOp {
+        dst: then_result,
+        op: BinOpKind::Add,
+        lhs: carrier_param,
+        rhs: then_const_id,
+    }));
+
+    // Step 4: Compute else_result = carrier + else_delta
+    let else_const_id = alloc_value();
+    instructions.push(JoinInst::Compute(MirLikeInst::Const {
+        dst: else_const_id,
+        value: ConstValue::Integer(else_delta),
+    }));
+    let else_result = alloc_value();
+    instructions.push(JoinInst::Compute(MirLikeInst::BinOp {
+        dst: else_result,
+        op: BinOpKind::Add,
+        lhs: carrier_param,
+        rhs: else_const_id,
+    }));
+
+    // Step 5: Emit Select instruction
+    let carrier_new: VarId = alloc_value();
+    instructions.push(JoinInst::Select {
+        dst: carrier_new,
+        cond: cond_id,
+        then_val: then_result,
+        else_val: else_result,
+        type_hint: Some(MirType::Integer),  // Carrier is always Integer
+    });
+
+    Ok(carrier_new)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/mir/join_ir/lowering/loop_with_break_minimal.rs

@@ -63,9 +63,10 @@ mod tests;
 
 use crate::mir::join_ir::lowering::carrier_info::{CarrierInfo, JoinFragmentMeta};
 use crate::mir::join_ir::lowering::carrier_update_emitter::{
-    emit_carrier_update, emit_carrier_update_with_env,
+    emit_carrier_update, emit_carrier_update_with_env, emit_conditional_step_update,
 };
 use crate::mir::join_ir::lowering::condition_to_joinir::ConditionEnv;
+use crate::mir::loop_canonicalizer::UpdateKind;
 use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
 use crate::mir::join_ir::lowering::loop_body_local_env::LoopBodyLocalEnv;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
@@ -145,6 +146,7 @@ use std::collections::BTreeMap; // Phase 222.5-D: HashMap → BTreeMap for deter
 /// * `body_ast` - Phase 191: Loop body AST for body-local init lowering
 /// * `body_local_env` - Phase 185-2: Optional mutable body-local variable environment for init expressions
 /// * `join_value_space` - Phase 201: Unified JoinIR ValueId allocator (Local region: 1000+)
+/// * `skeleton` - Phase 92 P0-3: Optional LoopSkeleton for ConditionalStep support
 pub(crate) fn lower_loop_with_break_minimal(
     _scope: LoopScopeShape,
     condition: &ASTNode,
@@ -155,6 +157,7 @@ pub(crate) fn lower_loop_with_break_minimal(
     body_ast: &[ASTNode],
     mut body_local_env: Option<&mut LoopBodyLocalEnv>,
     join_value_space: &mut JoinValueSpace,
+    skeleton: Option<&crate::mir::loop_canonicalizer::LoopSkeleton>,
 ) -> Result<(JoinModule, JoinFragmentMeta), String> {
     // Phase 170-D-impl-3: Validate that conditions only use supported variable scopes
     // LoopConditionScopeBox checks that loop conditions don't reference loop-body-local variables
@@ -585,6 +588,34 @@ pub(crate) fn lower_loop_with_break_minimal(
             continue;
         }
 
+        // Phase 92 P0-3: Check if skeleton has ConditionalStep for this carrier
+        if let Some(skel) = skeleton {
+            if let Some(carrier_slot) = skel.carriers.iter().find(|c| c.name == *carrier_name) {
+                if let UpdateKind::ConditionalStep { cond, then_delta, else_delta } = &carrier_slot.update_kind {
+                    // Use emit_conditional_step_update instead of emit_carrier_update
+                    eprintln!(
+                        "[joinir/pattern2] Phase 92 P0-3: ConditionalStep detected for carrier '{}': then={}, else={}",
+                        carrier_name, then_delta, else_delta
+                    );
+                    let updated_value = emit_conditional_step_update(
+                        carrier,
+                        &*cond,
+                        *then_delta,
+                        *else_delta,
+                        &mut alloc_value,
+                        env,
+                        &mut carrier_update_block,
+                    )?;
+                    updated_carrier_values.push(updated_value);
+                    eprintln!(
+                        "[joinir/pattern2] Phase 92 P0-3: ConditionalStep carrier '{}' updated → {:?}",
+                        carrier_name, updated_value
+                    );
+                    continue; // Skip normal carrier update
+                }
+            }
+        }
+
         // Get the update expression for this carrier
         let update_expr = carrier_updates.get(carrier_name).ok_or_else(|| {
             format!(

src/mir/join_ir/lowering/loop_with_break_minimal/tests.rs

@@ -144,6 +144,7 @@ fn test_pattern2_header_condition_via_exprlowerer() {
         &[],
         None,
         &mut join_value_space,
+        None, // Phase 92 P0-3: skeleton=None for backward compatibility
     );
 
     assert!(result.is_ok(), "ExprLowerer header path should succeed");

src/mir/join_ir/normalized/fixtures.rs

@@ -132,6 +132,7 @@ pub fn build_pattern2_minimal_structured() -> JoinModule {
         &[],
         None,
         &mut join_value_space,
+        None, // Phase 92 P0-3: skeleton=None for backward compatibility
     )
     .expect("pattern2 minimal lowering should succeed");
 

src/mir/loop_canonicalizer/canonicalizer.rs

@@ -314,7 +314,8 @@ pub fn canonicalize_loop_expr(
     // Chosen: Pattern2Break (same as skip_whitespace, but with richer Skeleton)
     // Notes: Added for parity/observability, lowering deferred to Phase 92
 
-    if let Some((counter_name, normal_delta, escape_delta, _quote_char, _escape_char, body_stmts)) =
+    // Phase 92 P0-3: Now also extracts escape_cond for JoinIR Select generation
+    if let Some((counter_name, normal_delta, escape_delta, _quote_char, _escape_char, body_stmts, escape_cond)) =
         try_extract_escape_skip_pattern(body)
     {
         // Build skeleton for escape skip pattern (P5b)
@@ -334,12 +335,14 @@ pub fn canonicalize_loop_expr(
 
         // Step 3: Update step with ConditionalStep (escape_delta vs normal_delta)
         // Pattern: normal i = i + 1, escape i = i + escape_delta (e.g., +2)
-        // Represented as UpdateKind::ConditionalStep with both deltas
+        // Represented as UpdateKind::ConditionalStep with both deltas and condition
+        // Phase 92 P0-3: Now includes escape_cond for JoinIR Select generation
         skeleton.steps.push(SkeletonStep::Update {
             carrier_name: counter_name.clone(),
             update_kind: UpdateKind::ConditionalStep {
-                then_delta: escape_delta,  // Escape branch: +2 or other
-                else_delta: normal_delta,   // Normal branch: +1
+                cond: escape_cond.clone(),  // Phase 92 P0-3: Condition for Select
+                then_delta: escape_delta,    // Escape branch: +2 or other
+                else_delta: normal_delta,    // Normal branch: +1
             },
         });
 
@@ -348,6 +351,7 @@ pub fn canonicalize_loop_expr(
             name: counter_name,
             role: CarrierRole::Counter,
             update_kind: UpdateKind::ConditionalStep {
+                cond: escape_cond,           // Phase 92 P0-3: Condition for Select
                 then_delta: escape_delta,
                 else_delta: normal_delta,
             },
@@ -1787,8 +1791,10 @@ mod tests {
         assert_eq!(carrier.role, CarrierRole::Counter, "Carrier should be a Counter");
 
         // Verify ConditionalStep with escape_delta=2, normal_delta=1
+        // Phase 92 P0-3: ConditionalStep now includes cond
         match &carrier.update_kind {
             UpdateKind::ConditionalStep {
+                cond: _,  // Phase 92 P0-3: Condition for Select (don't check exact AST)
                 then_delta,
                 else_delta,
             } => {

src/mir/loop_canonicalizer/pattern_recognizer.rs

@@ -310,16 +310,21 @@ mod tests {
 /// }
 /// ```
 ///
-/// Returns (counter_name, normal_delta, escape_delta, quote_char, escape_char, body_stmts)
+/// Returns (counter_name, normal_delta, escape_delta, quote_char, escape_char, body_stmts, escape_cond)
 /// if pattern matches.
 ///
 /// # Phase 91 P5b: Escape Sequence Pattern Detection
 ///
 /// This function delegates to `ast_feature_extractor::detect_escape_skip_pattern`
 /// for SSOT implementation.
+///
+/// # Phase 92 P0-3: Added escape_cond
+///
+/// The escape_cond is the condition expression for the conditional increment
+/// (e.g., `ch == '\\'`). This is needed for JoinIR Select generation.
 pub fn try_extract_escape_skip_pattern(
     body: &[ASTNode],
-) -> Option<(String, i64, i64, char, char, Vec<ASTNode>)> {
+) -> Option<(String, i64, i64, char, char, Vec<ASTNode>, Box<ASTNode>)> {
     ast_detect_escape(body).map(|info| {
         (
             info.counter_name,
@@ -328,6 +333,7 @@ pub fn try_extract_escape_skip_pattern(
             info.quote_char,
             info.escape_char,
             info.body_stmts,
+            info.escape_cond,  // Phase 92 P0-3: Condition for JoinIR Select
         )
     })
 }

src/mir/loop_canonicalizer/skeleton_types.rs

@@ -106,7 +106,15 @@ pub enum UpdateKind {
     /// ```
     ///
     /// Phase 91 P5b: Used for escape sequence handling and similar conditional increments
-    ConditionalStep { then_delta: i64, else_delta: i64 },
+    /// Phase 92 P0-3: Added condition expression for JoinIR Select generation
+    ConditionalStep {
+        /// The condition expression (e.g., `ch == '\\'`)
+        cond: Box<ASTNode>,
+        /// Delta for then branch (when condition is true)
+        then_delta: i64,
+        /// Delta for else branch (when condition is false)
+        else_delta: i64,
+    },
 
     /// Conditional update with AST expressions (`if cond { x = a } else { x = b }`)
     Conditional {