feat(joinir): Phase 171-172 Issue 4 - LoopScopeShapeBuilder unified initialization

Eliminates 50-60 lines of duplicated LoopScopeShape initialization across all 4 patterns.

**Changes**:
- New: `loop_scope_shape_builder.rs` with factory methods
- `empty_body_locals()`: For Pattern 1, 3 (condition-only analysis)
- `with_body_locals()`: For Pattern 2, 4 (AST-based body variable extraction)
- Updated all 4 patterns to use unified builder
- Includes unit tests for both factory methods

**Impact**:
- Pattern 1: 11 lines → 6 lines (5-line reduction)
- Pattern 2: 18 lines → 8 lines (10-line reduction)
- Pattern 3: 11 lines → 6 lines (5-line reduction)
- Pattern 4: 18 lines → 8 lines (10-line reduction)
- Total: 58 lines → 28 lines (30-line reduction, ~52%)

**Test**:
- cargo build --release: ✅ PASS
- cargo test --release: ✅ 720/800 PASS (same as before)
- Unit tests: ✅ 3/3 PASS

Phase 171-172 Stage 1: 25-30% deletion target (30 lines achieved)

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

@@ -23,11 +23,15 @@
 //! Phase 33-22: Common Pattern Infrastructure
 //! - common_init.rs: CommonPatternInitializer for unified initialization
 //! - conversion_pipeline.rs: JoinIRConversionPipeline for unified conversion flow
+//!
+//! Phase 171-172: Refactoring Infrastructure
+//! - loop_scope_shape_builder.rs: Unified LoopScopeShape initialization (Issue 4)
 
 pub(in crate::mir::builder) mod ast_feature_extractor;
 pub(in crate::mir::builder) mod common_init;
 pub(in crate::mir::builder) mod conversion_pipeline;
 pub(in crate::mir::builder) mod exit_binding;
+pub(in crate::mir::builder) mod loop_scope_shape_builder;
 pub(in crate::mir::builder) mod pattern1_minimal;
 pub(in crate::mir::builder) mod pattern2_with_break;
 pub(in crate::mir::builder) mod pattern3_with_if_phi;