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/phase131-11-case-c-summary.md

@@ -19,7 +19,8 @@
   - PHI/型デバッグ: `docs/reference/environment-variables.md` の `NYASH_PHI_TYPE_DEBUG` / `NYASH_PHI_META_DEBUG`
 
 現状メモ:
-- VM では期待値に一致するが、LLVM では結果が一致しないケースが残っている（別トピックとして棚卸し/切り分けが必要）。
+- `apps/tests/llvm_stage3_loop_only.hako` については Phase 132-P2 で VM/LLVM parity（`Result: 3`）まで到達した。
+  - 調査ログ: `docs/development/current/main/investigations/phase132-case-c-llvm-exe.md`
 
 ---