feat(joinir): Add TrimLoopLowerer skeleton for P5 module

Phase 180-2: Create dedicated Trim/CharComparison lowering module

- New module: src/mir/join_ir/lowering/trim_loop_lowering.rs
- TrimLoopLowerer::try_lower_trim_like_loop() skeleton
- Integrates LoopConditionScopeBox + LoopBodyCarrierPromoter
- Returns TrimLoweringResult with updated condition/carrier/bindings
- TODO: Phase 180-3 will implement full logic from Pattern2

src/mir/join_ir/lowering/mod.rs

@@ -60,6 +60,7 @@ pub mod skip_ws;
 pub mod stage1_using_resolver;
 pub mod stageb_body;
 pub mod stageb_funcscanner;
+pub mod trim_loop_lowering; // Phase 180: Trim/P5 dedicated lowering module
 pub mod type_hint_policy; // Phase 65.5: 型ヒントポリシー箱化
 pub mod type_inference; // Phase 65-2-A
 pub(crate) mod value_id_ranges; // Internal ValueId range management