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/loop_pattern_detection/condition_var_analyzer.rs

@@ -124,7 +124,7 @@ pub fn extract_all_variables(node: &ASTNode) -> HashSet<String> {
 /// Future versions may include:
 /// - Dominance tree analysis
 /// - More sophisticated scope inference
-pub fn is_outer_scope_variable(
+pub(crate) fn is_outer_scope_variable(
     var_name: &str,
     scope: Option<&LoopScopeShape>,
 ) -> bool {