feat(joinir): Phase 183 LoopBodyLocal role separation + test fixes

Phase 183 Implementation:
- Added is_var_used_in_condition() helper for AST variable detection
- Implemented LoopBodyLocal filtering in TrimLoopLowerer
- Created 4 test files for P1/P2 patterns
- Added 5 unit tests for variable detection

Test Fixes:
- Fixed test_is_outer_scope_variable_pinned (BasicBlockId import)
- Fixed test_pattern2_accepts_loop_param_only (literal node usage)

Refactoring:
- Unified pattern detection documentation
- Consolidated CarrierInfo initialization
- Documented LoopScopeShape construction paths

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -71,8 +71,6 @@ pub use funcscanner_trim::lower_funcscanner_trim_to_joinir;
 pub use inline_boundary_builder::JoinInlineBoundaryBuilder;
 // Phase 31: LoopToJoinLowerer 統一箱
 pub use loop_to_join::LoopToJoinLowerer;
-// Phase 188: Pattern-based loop lowering
-pub use loop_with_if_phi_minimal::lower_loop_with_if_phi_pattern;
 // Phase 30 F-3: 旧 lower_case_a_loop_to_joinir_for_minimal_skip_ws は _with_scope に置き換え済みのため削除
 pub use min_loop::lower_min_loop_to_joinir;
 pub use skip_ws::lower_skip_ws_to_joinir;