feat(joinir): Phase 213-2 Step 2-2 & 2-3 Data structure extensions

Extended PatternPipelineContext and CarrierUpdateInfo for Pattern 3 AST-based generalization.

Changes:
1. PatternPipelineContext:
   - Added loop_condition: Option<ASTNode>
   - Added loop_body: Option<Vec<ASTNode>>
   - Added loop_update_summary: Option<LoopUpdateSummary>
   - Updated build_pattern_context() for Pattern 3

2. CarrierUpdateInfo:
   - Added then_expr: Option<ASTNode>
   - Added else_expr: Option<ASTNode>
   - Updated analyze_loop_updates() with None defaults

Status: Phase 213-2 Steps 2-2 & 2-3 complete
Next: Create Pattern3IfAnalyzer to extract if statement and populate update summary

src/mir/builder/loop_frontend_binding.rs

@@ -153,6 +153,7 @@ impl LoopFrontendBinding {
     /// - Bound: constant 3
     /// - No accumulator (side-effect only loop)
     /// - No external refs
+    #[allow(dead_code)]
     pub fn for_main_simple_while() -> Self {
         Self {
             counter_var: "i".to_string(),