feat(joinir): Phase 35-5 PHI box deletion (430 lines HIGH safety)

Phase 35-5: First wave deletion
- Deleted if_body_local_merge.rs (339 lines, 0 external calls)
  - Inlined logic into PhiBuilderBox::compute_modified_names_if()
  - Absorbed into PhiBuilderBox (isolated box with no external dependencies)
- Deleted phi_invariants.rs (91 lines, moved to JoinIR Verifier)
  - Removed ensure_if_values_exist() call from phi_builder_box.rs
  - Validation responsibility transferred to JoinIR Verifier
- Updated PHI_BOX_INVENTORY.md with deletion records

Phase 35-3/4: Documentation and route unification
- Added frontend_covered column to PHI_BOX_INVENTORY.md
- Documented JoinIR Runner routes (Route A: SSOT, Route B: structure validation)
- Updated join_ir_runner.rs module docstring with clear route guidelines

Build and test status:
✅ cargo build --release: Clean
✅ Phase 34 tests: All PASS (no regression)
✅ JoinIR tests: joinir_frontend_if_select, test_if_merge_simple_pattern PASS

Total reduction: 430 lines (HIGH safety achieved)
Phase 36+ potential: ~3,275 lines (MEDIUM/LOW safety)

Related docs:
- docs/private/roadmap2/phases/phase-35-phi-reduction/README.md
- docs/private/roadmap2/phases/phase-35-phi-reduction-investigation-report.md
- docs/private/roadmap2/phases/phase-30-final-joinir-world/PHI_BOX_INVENTORY.md

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

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

src/mir/loop_builder.rs

@@ -1168,7 +1168,8 @@ impl<'a> LoopBuilder<'a> {
 
         let mut ops = Ops(self);
 
-        // Phase 26-F-2: BodyLocalPhiBuilder削除、IfBodyLocalMergeBox使用
+        // Phase 26-F-2: BodyLocalPhiBuilder削除
+        // Phase 35-5: if_body_local_merge.rs削除、PhiBuilderBoxに吸収
         // 理由: 箱理論による責務分離（ループスコープ分析 vs if-merge専用処理）
 
         // Phase 26-E: PhiBuilderBox SSOT統合（If PHI生成）
@@ -1189,7 +1190,7 @@ impl<'a> LoopBuilder<'a> {
             carrier_names,
         );
 
-        // Phase 26-F-2: IfBodyLocalMergeBox は phi_builder_box.rs 内で直接使用
+        // Phase 35-5: if_body_local_merge.rs削除、ロジックはPhiBuilderBox内に統合
 
         let post_snapshots = if let Some(ref else_map) = else_var_map_end_opt {
             vec![then_var_map_end.clone(), else_map.clone()]