feat(joinir): Phase 171 complete - Trim pattern LoopBodyLocal promotion

Phase 171-C-4/5 + impl-Trim: Full Trim pattern validation infrastructure

## CarrierInfo 統合 (C-4)
- CarrierInfo::merge_from(): Deduplicated carrier merging
- TrimPatternInfo::to_carrier_info(): Conversion helper
- Pattern2/4: Promoted carrier merge integration
- 7 unit tests for merge logic

## TrimLoopHelper 設計 (C-5)
- TrimLoopHelper struct: Trim-specific validation box
- carrier_type(), initial_value(), whitespace helpers
- CarrierInfo::trim_helper() accessor
- 5 unit tests

## Validation-Only Integration (impl-Trim)
- TrimLoopHelper::is_safe_trim(), is_trim_like(), has_valid_structure()
- Pattern2/4: Trim exception route with safety validation
- body_locals extraction from loop body AST
- LoopBodyCarrierPromoter: ASTNode::Local handler extension
- 4 unit tests for safety validation

## Architecture
- Box Theory: TrimLoopHelper is "validation only" (no JoinIR generation)
- Fail-Fast: Non-Trim LoopBodyLocal immediately rejected
- Whitelist approach: Only Trim pattern bypasses LoopBodyLocal restriction

Tests: 16 new unit tests, all passing
E2E: test_trim_main_pattern.hako validation successful

Next: Phase 172 - Actual JoinIR lowering for Trim pattern

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

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

src/mir/loop_pattern_detection/mod.rs

@@ -761,3 +761,7 @@ pub mod error_messages;
 
 // Phase 171-C: LoopBodyLocal Carrier Promotion
 pub mod loop_body_carrier_promoter;
+
+// Phase 171-C-5: Trim Pattern Helper
+pub mod trim_loop_helper;
+pub use trim_loop_helper::TrimLoopHelper;