refactor(joinir): Phase 287 P0.1 - Move verification to debug_assertions

- Move verify_no_phi_dst_overwrite() to debug_assertions.rs
- Move verify_phi_inputs_defined() to debug_assertions.rs
- Move verify_joinir_contracts() to debug_assertions.rs
- Remove duplicate get_instruction_dst() from mod.rs
- mod.rs: 1,555 → ~1,380 lines (-176 lines)
- Semantic invariance: 154/154 smoke tests PASS, Pattern6 RC:9

Phase 287 P0: Big Files Refactoring (意味論不変)

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

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

docs/development/current/main/10-Now.md

@@ -8,6 +8,7 @@
 - `./tools/smokes/v2/run.sh --profile quick` 154/154 PASS 維持
 - 入口: `docs/development/current/main/phases/phase-188.3/README.md`
 - 次の指示書（refactor挟み込み）: `docs/development/current/main/phases/phase-188.3/P2-REFACTORING-INSTRUCTIONS.md`
+- 次の指示書（でかいファイル分割）: `docs/development/current/main/phases/phase-287/P0-BIGFILES-REFACTORING-INSTRUCTIONS.md`
 
 **2025-12-27: Phase 188.2 完了** ✅
 - StepTreeの `max_loop_depth` を SSOT に採用（Option A）