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/instance_v2.rs

@@ -436,7 +436,7 @@ impl Display for InstanceBox {
 mod tests {
     use super::*;
     use crate::box_trait::{IntegerBox, NyashBox};
-    use std::sync::{Arc, Mutex};
+    use std::sync::Arc;
     type SharedNyashBox = Arc<dyn NyashBox>;
 
     #[test]