refactor: Extract JoinIR routing logic from control_flow.rs (Phase 3)

- Created joinir/routing.rs
- Moved try_cf_loop_joinir() and cf_loop_joinir_impl()
- Updated joinir/mod.rs to include routing module
- Removed ~320 lines from main control_flow.rs
- Zero breaking changes, all tests pass

Phase 1-3 complete:
- control_flow.rs: 1,632 → ~900 lines (45% reduction)
- Extracted 3 modules: debug, patterns (3 files), routing
- All functionality preserved and verified

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

@@ -2,6 +2,7 @@
 //!
 //! This module contains JoinIR-related control flow logic:
 //! - Pattern lowerers (patterns/)
-//! - Routing logic (future Phase 3)
+//! - Routing logic (routing.rs) ✅
 
 pub(in crate::mir::builder) mod patterns;
+pub(in crate::mir::builder) mod routing;