docs(joinir): Phase 187 LoopBuilder Physical Removal - Completion Documentation

## Changes
- Added Phase 187 section to Phase 180 README documenting complete LoopBuilder deletion
- Documented all 4 tasks completion (187-1 through 187-4)
- Added architectural impact analysis before/after Phase 187
- Documented code deletion summary and archival strategy
- Explained NYASH_LEGACY_LOOPBUILDER variable status after deletion

## Key Achievements
- LoopBuilder module (8 files, ~1000+ lines) completely deleted
- IfInLoopPhiEmitter preserved in minimal new module
- JoinIR Frontend is now sole authoritative loop lowering system
- Explicit failures replace implicit fallbacks

## Architectural Impact
- Single authoritative path (JoinIR Frontend)
- No implicit fallbacks
- Future JoinIR expansion is only way forward
- Fail-Fast principle enforced at architecture level

## Related Phases
- Phase 185: Strict mode semantics unified
- Phase 186: Hard freeze with access control guard
- Phase 187: Physical module deletion (this change)

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

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

src/mir/mod.rs

@@ -16,7 +16,7 @@ pub mod instruction;
 pub mod instruction_introspection; // Introspection helpers for tests (instruction names)
 pub mod instruction_kinds; // small kind-specific metadata (Const/BinOp)
 pub mod loop_api; // Minimal LoopBuilder facade (adapter-ready)
-pub mod loop_builder; // SSA loop construction with phi nodes
+pub mod if_in_loop_phi; // Phase 187-2: Minimal if-in-loop PHI emitter (extracted from loop_builder)
 pub mod naming; // Static box / entry naming rules（NamingBox）
 pub mod optimizer;
 pub mod ssot; // Shared helpers (SSOT) for instruction lowering