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/control_flow.rs

@@ -57,19 +57,14 @@ impl super::MirBuilder {
             eprintln!("[cf_loop] Current stack (simulated): check build_statement vs build_expression_impl");
         }
 
-        // Phase 186: LoopBuilder Hard Freeze - Require explicit legacy mode opt-in
-        if !crate::config::env::joinir_dev::legacy_loopbuilder_enabled() {
-            return Err(format!(
-                "[joinir/freeze] Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder is disabled.\n\
-                 Function: {}\n\
-                 Hint: Set NYASH_LEGACY_LOOPBUILDER=1 to enable legacy path (dev-only)",
-                self.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
-            ));
-        }
-
-        // Delegate to LoopBuilder for consistent handling (legacy path only)
-        let mut loop_builder = crate::mir::loop_builder::LoopBuilder::new(self);
-        loop_builder.build_loop(condition, body)
+        // Phase 186: LoopBuilder Hard Freeze - Legacy path disabled
+        // Phase 187-2: LoopBuilder module removed - all loops must use JoinIR
+        return Err(format!(
+            "[joinir/freeze] Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder has been removed.\n\
+             Function: {}\n\
+             Hint: This loop pattern is not supported. All loops must use JoinIR lowering.",
+            self.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
+        ));
     }
 
     /// Phase 49: Try JoinIR Frontend for mainline integration