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) => {