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/patterns/router.rs

@@ -184,9 +184,10 @@ pub fn route_loop_pattern(
         }
     }
 
-    // No pattern matched - fall through to legacy path
+    // No pattern matched - return None (caller will handle error)
+    // Phase 187-2: Legacy LoopBuilder removed, all loops must use JoinIR
     if ctx.debug {
-        trace::trace().debug("route", &format!("No pattern matched for function '{}', falling back to legacy", ctx.func_name));
+        trace::trace().debug("route", &format!("No pattern matched for function '{}'", ctx.func_name));
     }
     Ok(None)
 }