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/loop_condition_scope.rs

@@ -131,7 +131,7 @@ impl LoopConditionScopeBox {
     ///    - If matches loop_param_name → LoopParam
     ///    - Else if in outer scope (via condition_var_analyzer) → OuterLocal
     ///    - Else → LoopBodyLocal (conservative default)
-    pub fn analyze(
+    pub(crate) fn analyze(
         loop_param_name: &str,
         condition_nodes: &[&ASTNode],
         scope: Option<&LoopScopeShape>,