feat(joinir): Phase 132-P3 - Exit PHI collision early detection

Added verify_exit_phi_no_collision() to contract_checks.rs for early detection
of ValueId collisions between exit PHIs and other instructions.

Problem detected:
- If exit_phi_builder uses builder.value_gen.next() (module-level) instead of
  func.next_value_id() (function-level), ValueIds can collide:

  Example:
  - bb0: %1 = const 0   (counter init)
  - bb3: %1 = phi ...   (exit PHI - collision!)

Previous behavior:
- Error only detected at LLVM backend runtime
- Cryptic error: "Cannot overwrite PHI dst=1"

New behavior:
- Panic at Rust compile time (debug build)
- Clear error message with fix suggestion:
  "Exit PHI dst %1 collides with instruction in block 0
   Fix: Use func.next_value_id() in exit_phi_builder.rs"

Benefits:
- 🔥 Fail-Fast: Catch errors during Rust compilation, not LLVM execution
- 📋 Clear messages: Exact collision point + fix suggestion
- 🧪 Testable: verify_exit_phi_no_collision() can be unit tested

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

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

docs/development/current/main/phases/phase-132/README.md

@@ -2,7 +2,7 @@
 
 **Date**: 2025-12-15  
 **Status**: ✅ Done  
-**Scope**: Pattern 1（Simple While）で「ループ終了後の `return i`」が VM/LLVM で一致することを固定する
+**Scope**: ループ exit 値（exit PHI / boundary）が VM/LLVM で一致することを固定する（Pattern 1 と Case C を含む）
 
 ---
 
@@ -64,3 +64,30 @@ static box Main {
 詳細ログ:
 - `docs/development/current/main/investigations/phase132-llvm-exit-phi-wrong-result.md`
 
+---
+
+## 追加: Phase 132-P2（Case C）Exit PHI ValueId collision fix
+
+Case C（`apps/tests/llvm_stage3_loop_only.hako`）の LLVM EXE 検証で、
+**exit PHI の ValueId 衝突**が原因で `Result: 0` になる問題が見つかった。
+
+### Root Cause
+
+- `src/mir/builder/control_flow/joinir/merge/exit_phi_builder.rs` が
+  PHI dst の割り当てに `builder.value_gen.next()`（module-level）を使っており、
+  同一関数内で ValueId が衝突し得た。
+
+### Fix
+
+- PHI dst の割り当てを `func.next_value_id()`（function-level）へ統一。
+
+コミット:
+- `bd07b7f4 fix(joinir): Phase 132-P2 - Exit PHI ValueId collision fix`
+
+### Verification
+
+- Pattern 1: VM/LLVM ともに `3`
+- Case C: VM/LLVM ともに `Result: 3`
+
+調査ログ:
+- `docs/development/current/main/investigations/phase132-case-c-llvm-exe.md`