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/patterns/mod.rs

@@ -0,0 +1,10 @@
+//! Pattern lowerers for different loop constructs
+//!
+//! Phase 2: Extracted from control_flow.rs
+//! - Pattern 1: Simple While Loop (pattern1_minimal.rs)
+//! - Pattern 2: Loop with Conditional Break (pattern2_with_break.rs)
+//! - Pattern 3: Loop with If-Else PHI (pattern3_with_if_phi.rs)
+
+pub(in crate::mir::builder) mod pattern1_minimal;
+pub(in crate::mir::builder) mod pattern2_with_break;
+pub(in crate::mir::builder) mod pattern3_with_if_phi;