refactor(joinir): Move Trim logic from Pattern2 to TrimLoopLowerer

Phase 180-3: Extract Pattern2 Trim/P5 logic to dedicated module

Changes:
- Pattern2: Delegated Trim processing to TrimLoopLowerer (~160 lines removed)
- Pattern2: Simplified to ~25 lines of delegation code
- TrimLoopLowerer: Implemented full logic from Pattern2 (lines 180-340)
- Net reduction: -135 lines in Pattern2 (371 lines total)

Implementation:
- LoopConditionScopeBox + LoopBodyCarrierPromoter integration
- Carrier initialization code generation (substring + whitespace check)
- Trim break condition replacement (!is_carrier)
- ConditionEnv bindings setup (carrier + original variable)

Testing:
- cargo build --release: SUCCESS (0 errors, warnings only)
- All existing Pattern2 tests: PASS
- No behavior changes, refactoring only

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

@@ -51,6 +51,7 @@ pub(in crate::mir::builder) mod pattern3_with_if_phi;
 pub(in crate::mir::builder) mod pattern4_carrier_analyzer;
 pub(in crate::mir::builder) mod pattern4_with_continue;
 pub(in crate::mir::builder) mod router;
+pub(in crate::mir::builder) mod trim_loop_lowering; // Phase 180: Dedicated Trim/P5 lowering module
 pub(in crate::mir::builder) mod trim_pattern_validator;
 pub(in crate::mir::builder) mod trim_pattern_lowerer;