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/phase131-3-llvm-lowering-inventory.md

@@ -10,7 +10,7 @@
 | A | `apps/tests/phase87_llvm_exe_min.hako` | ✅ | ✅ | ✅ | **PASS** - Simple return 42, no BoxCall, exit code verified |
 | B | `apps/tests/loop_min_while.hako` | ✅ | ✅ | ✅ | **PASS** - Loop/PHI path runs end-to-end (Phase 131-10): prints `0,1,2` and exits |
 | B2 | `/tmp/case_b_simple.hako` | ✅ | ✅ | ✅ | **PASS** - Simple print(42) without loop works |
-| C | `apps/tests/llvm_stage3_loop_only.hako` | ✅ | ✅ | ⚠️ | **TAG-RUN** - Loop ok; print/concat path segfaults |
+| C | `apps/tests/llvm_stage3_loop_only.hako` | ✅ | ✅ | ✅ | **PASS** - `loop(true)` + break/continue + print/concat works end-to-end (Phase 132-P2) |
 
 ## Phase 132 Update (2025-12-15)