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/builder/if_form.rs

@@ -280,7 +280,7 @@ impl MirBuilder {
                             // PHI 生成
                             let phi_count = {
                                 let mut ops = ToplevelOps(self);
-                                crate::mir::loop_builder::IfInLoopPhiEmitter::emit_toplevel_phis(
+                                crate::mir::if_in_loop_phi::IfInLoopPhiEmitter::emit_toplevel_phis(
                                     &phi_spec,
                                     &pre_if_var_map,
                                     &then_var_map_end,