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/control_flow/mod.rs

@@ -86,9 +86,8 @@ impl super::MirBuilder {
     ///
     /// This is the unified entry point for all loop lowering. All loops are processed
     /// via JoinIR Frontend (Phase 187-2: LoopBuilder removed).
-    /// Specific functions are enabled via dev flags (Phase 49) or Core policy (Phase 80):
+    /// Specific functions are enabled via dev flags (Phase 49):
     ///
-    /// - Core ON (`joinir_core_enabled()`): print_tokens / ArrayExt.filter はまず JoinIR Frontend を試す
     /// - Dev フラグ（既存）:
     ///   - `HAKO_JOINIR_PRINT_TOKENS_MAIN=1`: JsonTokenizer.print_tokens/0
     ///   - `HAKO_JOINIR_ARRAY_FILTER_MAIN=1`: ArrayExtBox.filter/2