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/if_in_loop_phi/mod.rs

@@ -364,7 +364,7 @@ mod tests {
         assert_eq!(count, 1);
         assert_eq!(ops.emitted_phis.len(), 1);
 
-        let (block, dst, inputs) = &ops.emitted_phis[0];
+        let (block, _dst, inputs) = &ops.emitted_phis[0];
         assert_eq!(*block, BasicBlockId(13)); // merge_block
         assert_eq!(inputs.len(), 2);
         assert_eq!(inputs[0], (BasicBlockId(11), ValueId(2))); // then: 変更値