refactor(joinir): Unify Pattern 3 with PatternPipelineContext

Phase 179-B Task 4: Refactor Pattern 3 to use PatternPipelineContext
for unified preprocessing.

Changes:
- Use build_pattern_context() for loop variable extraction and scope construction
- Extract sum_var_id from ctx.carrier_info instead of direct initialization
- Reduce line count: 168 → 149 lines (11% reduction, lower than target due to already optimized code)
- Maintain exact same behavior and test compatibility

Benefits:
- Consistent preprocessing logic with Pattern 1
- Single source of truth for carrier analysis
- Cleaner separation of concerns (analysis vs lowering)

🤖 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/pattern3_with_if_phi.rs

@@ -50,44 +50,39 @@ pub fn lower(
 }
 
 impl MirBuilder {
-    /// Phase 188-Impl-3: Pattern 3 (Loop with If-Else PHI) minimal lowerer
+    /// Phase 179-B: Pattern 3 (Loop with If-Else PHI) minimal lowerer
     ///
-    /// Handles loops with if-else statements that assign to carrier variables.
+    /// **Refactored**: Now uses PatternPipelineContext for unified preprocessing
     ///
-    /// # Steps
-    /// 1. Extract loop variables (carriers: i + sum)
-    /// 2. Generate JoinIR using loop_with_if_phi_minimal
-    /// 3. Convert JoinModule → MirModule
-    /// 4. Create JoinInlineBoundary for input mapping
-    /// 5. Merge MIR blocks into current_function
-    /// 6. Return Void (loop doesn't produce values)
+    /// # Pipeline (Phase 179-B)
+    /// 1. Build preprocessing context → PatternPipelineContext
+    /// 2. Call JoinIR lowerer → JoinModule
+    /// 3. Create boundary from context → JoinInlineBoundary
+    /// 4. Merge MIR blocks → JoinIRConversionPipeline
     pub(in crate::mir::builder) fn cf_loop_pattern3_with_if_phi(
         &mut self,
         condition: &ASTNode,
-        _body: &[ASTNode],
+        body: &[ASTNode],
         _func_name: &str,
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_if_phi_minimal::lower_loop_with_if_phi_pattern;
-        use crate::mir::BasicBlockId;
-        use std::collections::BTreeSet;
 
         // Phase 195: Use unified trace
         trace::trace().debug("pattern3", "Calling Pattern 3 minimal lowerer");
 
-        // Phase 33-22: Use CommonPatternInitializer for loop variable extraction
-        use super::common_init::CommonPatternInitializer;
-        let (loop_var_name, loop_var_id, carrier_info) =
-            CommonPatternInitializer::initialize_pattern(
-                self,
-                condition,
-                &self.variable_map,
-                None, // Pattern 3 includes all carriers (i + sum)
-            )?;
+        // Phase 179-B: Use PatternPipelineContext for unified preprocessing
+        use super::pattern_pipeline::{build_pattern_context, PatternVariant};
+        let ctx = build_pattern_context(
+            self,
+            condition,
+            body,
+            PatternVariant::Pattern3,
+        )?;
 
-        // Phase 33-22: Extract sum_var_id from carrier_info
+        // Phase 179-B: Extract sum_var_id from context
         // Pattern 3 specifically needs the "sum" carrier
-        let sum_var_id = carrier_info.carriers.iter()
+        let sum_var_id = ctx.carrier_info.carriers.iter()
             .find(|c| c.name == "sum")
             .ok_or_else(|| {
                 format!(
@@ -99,19 +94,8 @@ impl MirBuilder {
         // Phase 195: Use unified trace
         trace::trace().varmap("pattern3_start", &self.variable_map);
 
-        // Phase 171-172: Use LoopScopeShapeBuilder for unified initialization (Issue 4)
-        // Pattern 3 lowerer ignores the scope anyway, so this is just a placeholder
-        use super::loop_scope_shape_builder::LoopScopeShapeBuilder;
-        let scope = LoopScopeShapeBuilder::empty_body_locals(
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BTreeSet::new(),
-        );
-
-        // Call Pattern 3 lowerer
-        let join_module = match lower_loop_with_if_phi_pattern(scope) {
+        // Call Pattern 3 lowerer with preprocessed scope
+        let join_module = match lower_loop_with_if_phi_pattern(ctx.loop_scope) {
             Some(module) => module,
             None => {
                 // Phase 195: Use unified trace
@@ -120,17 +104,16 @@ impl MirBuilder {
             }
         };
 
-        // Phase 33-22: Create boundary for JoinIR conversion
+        // Phase 179-B: Create boundary from context
         // Phase 201: Use JoinInlineBoundaryBuilder for clean construction
         // Canonical Builder pattern - see docs/development/current/main/joinir-boundary-builder-pattern.md
-        // Pattern 3 has TWO carriers: i and sum
         self.trace_varmap("pattern3_before_merge");
         use crate::mir::join_ir::lowering::JoinInlineBoundaryBuilder;
         use crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding;
         let boundary = JoinInlineBoundaryBuilder::new()
             .with_inputs(
                 vec![ValueId(0), ValueId(1)],      // JoinIR's main() parameters (i, sum init)
-                vec![loop_var_id, sum_var_id],     // Host's loop variables
+                vec![ctx.loop_var_id, sum_var_id], // Host's loop variables
             )
             .with_exit_bindings(vec![
                 // Phase 33-16: Only include non-loop-variable carriers in exit_bindings
@@ -141,12 +124,12 @@ impl MirBuilder {
                     host_slot: sum_var_id,                            // variable_map["sum"]
                 }
             ])
-            .with_loop_var_name(Some(loop_var_name.clone()))  // Phase 33-16: Enable header PHI generation for SSA correctness
+            .with_loop_var_name(Some(ctx.loop_var_name.clone()))  // Phase 33-16: Enable header PHI generation for SSA correctness
             .build();
 
         // Phase 33-22: Use JoinIRConversionPipeline for unified conversion flow
         use super::conversion_pipeline::JoinIRConversionPipeline;
-        let _exit_phi_result = JoinIRConversionPipeline::execute(
+        let _ = JoinIRConversionPipeline::execute(
             self,
             join_module,
             Some(&boundary),
@@ -155,9 +138,7 @@ impl MirBuilder {
         )?;
         self.trace_varmap("pattern3_after_merge");
 
-        // Phase 189-Refine: variable_map の更新は merge_joinir_mir_blocks 内で
-        // JoinInlineBoundary.host_outputs を用いて行われる。
-        // この関数では Void を返すだけでよい（戻り値は後続の `return sum` が扱う）。
+        // Phase 179-B: Return Void (loop doesn't produce values)
         let void_val = crate::mir::builder::emission::constant::emit_void(self);
 
         // Phase 195: Use unified trace