refactor: Extract pattern lowerers from control_flow.rs (Phase 2)

- Created joinir/patterns/ subdirectory
- Extracted Pattern 1: pattern1_minimal.rs (Simple While)
- Extracted Pattern 2: pattern2_with_break.rs (Loop with Break)
- Extracted Pattern 3: pattern3_with_if_phi.rs (Loop with If-Else PHI)
- Created patterns/mod.rs dispatcher
- Removed ~410 lines from main control_flow.rs
- Zero breaking changes, all tests pass

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

@@ -0,0 +1,7 @@
+//! JoinIR integration for control flow
+//!
+//! This module contains JoinIR-related control flow logic:
+//! - Pattern lowerers (patterns/)
+//! - Routing logic (future Phase 3)
+
+pub(in crate::mir::builder) mod patterns;