feat(joinir): Phase 184 - Body-local MIR Lowering Infrastructure

Phase 184 implements the foundation for body-local variable support in
update expressions, completing the three-box architecture:
LoopBodyLocalEnv (storage), UpdateEnv (composition), and
CarrierUpdateEmitter (emission).

## Implementation Summary

### Task 184-1: Design Document
- Created phase184-body-local-mir-lowering.md
- Two-Environment System design (ConditionEnv + LoopBodyLocalEnv)
- Box-First design principles documented

### Task 184-2: LoopBodyLocalEnv Implementation
- New file: src/mir/join_ir/lowering/loop_body_local_env.rs (216 lines)
- Storage box for body-local variable name → ValueId mappings
- BTreeMap for deterministic ordering (PHI consistency)
- 7 unit tests: empty env, single/multiple locals, get/contains, iteration

### Task 184-3: UpdateEnv Implementation
- New file: src/mir/join_ir/lowering/update_env.rs (237 lines)
- Composition box for unified variable resolution
- Priority order: ConditionEnv (condition vars) → LoopBodyLocalEnv (body-local)
- 8 unit tests: priority, fallback, not found, combined lookup

### Task 184-4: CarrierUpdateEmitter Integration
- Modified: src/mir/join_ir/lowering/carrier_update_emitter.rs
- Added emit_carrier_update_with_env() (UpdateEnv version)
- Kept emit_carrier_update() for backward compatibility
- 4 new unit tests: body-local variable, priority, not found, const update
- Total 10 tests PASS (6 existing + 4 new)

### Task 184-5: Representative Test Cases
- apps/tests/phase184_body_local_update.hako (Pattern1 baseline)
- apps/tests/phase184_body_local_with_break.hako (Pattern2, Phase 185 target)

### Task 184-6: Documentation Updates
- Updated: docs/development/current/main/joinir-architecture-overview.md
- Updated: CURRENT_TASK.md (Phase 184 completion record)

## Test Results

All 25 unit tests PASS:
- LoopBodyLocalEnv: 7 tests
- UpdateEnv: 8 tests
- CarrierUpdateEmitter: 10 tests (6 existing + 4 new)

Build: ✅ Success (0 errors)

## Design Constraints

Following 箱理論 (Box Theory) principles:
- Single Responsibility: Each box has one clear purpose
- Deterministic: BTreeMap ensures consistent ordering
- Conservative: Pattern5 (Trim) integration deferred to Phase 185
- Fail-Fast: Explicit errors for unsupported patterns

## Scope Limitation

Phase 184 provides the **infrastructure only**:
- ✅ Storage box (LoopBodyLocalEnv)
- ✅ Composition box (UpdateEnv)
- ✅ Emission support (CarrierUpdateEmitter)
- ❌ Pattern2/4 integration (requires body-local collection, Phase 185)

## Next Steps

Phase 185: Pattern2/4 Integration
- Integrate LoopBodyLocalEnv into Pattern2/4 lowerers
- Add body-local variable collection from loop body AST
- Enable full E2E body-local variable support in update expressions

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

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

src/mir/join_ir/lowering/mod.rs

@@ -24,6 +24,7 @@ pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub(crate) mod carrier_update_emitter; // Phase 179: Carrier update instruction emission
 pub(crate) mod common; // Internal lowering utilities
 pub mod condition_env; // Phase 171-fix: Condition expression environment
+pub mod loop_body_local_env; // Phase 184: Body-local variable environment
 pub(crate) mod condition_lowerer; // Phase 171-fix: Core condition lowering logic
 pub mod condition_to_joinir; // Phase 169: JoinIR condition lowering orchestrator (refactored)
 pub(crate) mod condition_var_extractor; // Phase 171-fix: Variable extraction from condition AST
@@ -62,6 +63,7 @@ pub mod stageb_body;
 pub mod stageb_funcscanner;
 pub mod type_hint_policy; // Phase 65.5: 型ヒントポリシー箱化
 pub mod type_inference; // Phase 65-2-A
+pub mod update_env; // Phase 184: Unified variable resolution for update expressions
 pub(crate) mod value_id_ranges; // Internal ValueId range management
 
 // Re-export public lowering functions