refactor(joinir): Extract legacy binding path to routing_legacy_binding.rs

Phase 179-A Step 2: Separate LoopFrontendBinding JSON construction logic
into dedicated module for better organization.

Changes:
- New file: routing_legacy_binding.rs (223 lines)
- routing.rs: cf_loop_joinir_impl() simplified to 15 lines (delegates to legacy path)
- Routing now clearly separates pattern-based vs. legacy binding paths

Benefits:
- Clear separation of concerns (pattern router vs. legacy whitelist)
- routing.rs reduced from 364 to 146 lines (60% reduction)
- Legacy path isolated for future deprecation

src/mir/builder/control_flow/joinir/mod.rs

@@ -8,5 +8,6 @@
 
 pub(in crate::mir::builder) mod patterns;
 pub(in crate::mir::builder) mod routing;
+pub(in crate::mir::builder) mod routing_legacy_binding;
 pub(in crate::mir::builder) mod merge;
 pub(in crate::mir::builder) mod trace;