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/basic_block.rs

@@ -479,7 +479,7 @@ mod tests {
     #[test]
     fn test_basic_block_creation() {
         let bb_id = BasicBlockId::new(0);
-        let mut bb = BasicBlock::new(bb_id);
+        let bb = BasicBlock::new(bb_id);
 
         assert_eq!(bb.id, bb_id);
         assert!(bb.is_empty());