refactor(joinir): Split ast_lowerer and join_ir_vm_bridge into modules

ast_lowerer.rs → ast_lowerer/ (10 files):
- mod.rs: public surface + entry dispatch
- context.rs: ExtractCtx helpers
- expr.rs: expression-to-JoinIR extraction
- if_return.rs: simple if→Select lowering
- loop_patterns.rs: loop variants (simple/break/continue)
- read_quoted.rs: read_quoted_from lowering (Phase 45-46)
- nested_if.rs: NestedIfMerge lowering
- analysis.rs: loop if-var analysis + metadata helpers
- tests.rs: frontend lowering tests
- README.md: module documentation

join_ir_vm_bridge.rs → join_ir_vm_bridge/ (5 files):
- mod.rs: public surface + shared helpers
- convert.rs: JoinIR→MIR lowering
- runner.rs: VM execution entry (run_joinir_via_vm)
- meta.rs: experimental metadata-aware hooks
- tests.rs: bridge-specific unit tests
- README.md: module documentation

Benefits:
- Clear separation of concerns per pattern
- Easier navigation and maintenance
- Each file has single responsibility
- README documents module boundaries

Co-authored-by: ChatGPT <noreply@openai.com>

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

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

src/mir/loop_builder.rs

@@ -300,8 +300,7 @@ impl<'a> LoopBuilder<'a> {
             .map(|f| f.signature.name.clone())
             .unwrap_or_default();
 
-        let bypass_flags =
-            crate::mir::phi_core::loopform_builder::get_loop_bypass_flags(&fn_name);
+        let bypass_flags = crate::mir::phi_core::loopform_builder::get_loop_bypass_flags(&fn_name);
 
         if bypass_flags.header {
             // Phase 27.4-C: JoinIR 実験経路では Header φ を生成しない。