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/10-Now.md

@@ -9,6 +9,12 @@
 - 結果: `/tmp/p1_return_i.hako` が 3 を返す（VM 一致）
 - 詳細: `investigations/phase132-llvm-exit-phi-wrong-result.md`
 
+**追加（Phase 132-P2）: Case C の Exit PHI ValueId 衝突を修正**
+- 原因: `exit_phi_builder.rs` が module-level allocator を使い、同一関数内で ValueId が衝突し得た
+- 修正: `func.next_value_id()`（function-level）へ統一（`bd07b7f4`）
+- 結果: `apps/tests/llvm_stage3_loop_only.hako` が LLVM EXE でも `Result: 3`（VM 一致）
+- 詳細: `investigations/phase132-case-c-llvm-exe.md`
+
 ## 2025‑12‑14：現状サマリ
 
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