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.rs

@@ -171,6 +171,7 @@ pub struct MirBuilder {
     /// Loop context stacks for lowering break/continue inside nested control flow
     /// Top of stack corresponds to the innermost active loop
     pub(super) loop_header_stack: Vec<BasicBlockId>,
+    #[allow(dead_code)]
     pub(super) loop_exit_stack: Vec<BasicBlockId>,
 
     /// If/merge context stack (innermost first). Used to make merge targets explicit
@@ -820,6 +821,7 @@ impl MirBuilder {
 
     /// Update an existing PHI instruction's inputs (for loop sealing)
     /// Used by LoopFormBuilder to complete incomplete PHI nodes
+    #[allow(dead_code)]
     pub(super) fn update_phi_instruction(
         &mut self,
         block: BasicBlockId,