refactor(joinir): Unify Pattern 4 with PatternPipelineContext

Phase 179-B Task 6: Refactor Pattern 4 to use PatternPipelineContext
for unified preprocessing.

Changes:
- Use build_pattern_context() for initial loop variable extraction and scope construction
- Extract loop_var_name, loop_var_id, carrier_info_prelim, and scope from context
- Keep Pattern4CarrierAnalyzer logic inline (Select-based continue semantics)
- Reduce line count: 422 → 414 lines (1.9% reduction)

Note:
Pattern 4 has complex carrier analysis (Select-based continue, carrier filtering,
normalization) that requires specialized Pattern4CarrierAnalyzer. The minimal
refactoring maintains this complexity while establishing the pipeline pattern.

Benefits:
- Consistent entry point with Patterns 1-3
- Unified preprocessing flow
- Maintains all existing functionality and test compatibility

🤖 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/pattern4_with_continue.rs

@@ -133,9 +133,16 @@ pub fn lower(
 }
 
 impl MirBuilder {
-    /// Phase 33-21: Pattern 4 (Loop with Continue) lowerer
+    /// Phase 179-B: Pattern 4 (Loop with Continue) lowerer
     ///
-    /// Handles loops with continue statements using Select-based carrier updates.
+    /// **Refactored**: Now uses PatternPipelineContext for unified preprocessing
+    ///
+    /// # Pipeline (Phase 179-B)
+    /// 1. Build preprocessing context → PatternPipelineContext
+    /// 2. Pattern 4-specific processing (continue normalization, carrier analysis)
+    /// 3. Call JoinIR lowerer → JoinModule
+    /// 4. Create boundary from context → JoinInlineBoundary
+    /// 5. Merge MIR blocks → JoinIRConversionPipeline
     ///
     /// # Example
     ///
@@ -152,15 +159,8 @@ impl MirBuilder {
     /// // sum = 25 (1+3+5+7+9)
     /// ```
     ///
-    /// # Implementation
-    ///
-    /// 1. Extract loop variable from condition
-    /// 2. Build CarrierInfo from variable_map
-    /// 3. Analyze carrier update expressions from loop body
-    /// 4. Generate JoinIR with Select-based continue semantics
-    /// 5. Convert JoinModule → MirModule
-    /// 6. Merge MIR blocks with header PHI and exit bindings
-    /// 7. Return Void (loop result accessed via variable_map)
+    /// Note: Pattern 4 has complex carrier analysis logic that remains inline
+    /// for now, using Pattern4CarrierAnalyzer for Select-based continue semantics.
     pub(in crate::mir::builder) fn cf_loop_pattern4_with_continue(
         &mut self,
         condition: &ASTNode,
@@ -169,8 +169,6 @@ impl MirBuilder {
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_continue_minimal::lower_loop_with_continue_minimal;
-        use crate::mir::BasicBlockId;
-        use std::collections::BTreeSet;
         use super::pattern4_carrier_analyzer::Pattern4CarrierAnalyzer;
 
         // Phase 195: Use unified trace
@@ -182,15 +180,21 @@ impl MirBuilder {
         let normalized_body = Pattern4CarrierAnalyzer::normalize_continue_branches(_body);
         let body_to_analyze = &normalized_body;
 
-        // Phase 33-22: Use CommonPatternInitializer for loop variable extraction
-        use super::common_init::CommonPatternInitializer;
-        let (loop_var_name, loop_var_id, carrier_info_prelim) =
-            CommonPatternInitializer::initialize_pattern(
-                self,
-                condition,
-                &self.variable_map,
-                None, // Pattern 4 will filter carriers via LoopUpdateAnalyzer
-            )?;
+        // Phase 179-B: Use PatternPipelineContext for unified preprocessing
+        // Note: Pattern 4 still needs inline processing for carrier analysis
+        use super::pattern_pipeline::{build_pattern_context, PatternVariant};
+        let ctx = build_pattern_context(
+            self,
+            condition,
+            body_to_analyze,
+            PatternVariant::Pattern4,
+        )?;
+
+        // Extract from context
+        let loop_var_name = ctx.loop_var_name.clone();
+        let loop_var_id = ctx.loop_var_id;
+        let carrier_info_prelim = ctx.carrier_info.clone();
+        let scope = ctx.loop_scope.clone();
 
         // Phase 33-23: Use Pattern4CarrierAnalyzer for carrier filtering
         // Analyze carrier update expressions FIRST to identify actual carriers
@@ -230,18 +234,6 @@ impl MirBuilder {
         // Phase 195: Use unified trace
         trace::trace().varmap("pattern4_start", &self.variable_map);
 
-        // Phase 171-172: Use LoopScopeShapeBuilder for unified initialization (Issue 4)
-        // Extract body_locals from loop body AST for proper variable classification
-        use super::loop_scope_shape_builder::LoopScopeShapeBuilder;
-        let scope = LoopScopeShapeBuilder::with_body_locals(
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BasicBlockId(0),
-            BTreeSet::new(),
-            body_to_analyze,
-        );
-
         // Phase 171-C-3: LoopBodyCarrierPromoter integration
         // Check if LoopConditionScopeBox detects LoopBodyLocal variables, and attempt promotion
         {