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

docs/guides/loopform.md

@@ -126,7 +126,7 @@ for / foreach の糖衣と正規化（概要）
  - 出力一致スモーク（VM, v2）
    - `tools/smokes/v2/run.sh --profile quick --filter "loop_two_vars|macro"`
  - 自己ホスト前展開（PyVM 経由）
-  - `NYASH_VM_USE_PY=1 NYASH_USE_NY_COMPILER=1 NYASH_MACRO_ENABLE=1 NYASH_MACRO_PATHS=apps/macros/examples/loop_normalize_macro.hako ./target/release/nyash --macro-preexpand --backend vm apps/tests/macro/loopform/simple.hako`
+  - `NYASH_VM_USE_PY=1 NYASH_USE_NY_COMPILER=1 NYASH_MACRO_ENABLE=1 NYASH_MACRO_PATHS=apps/macros/examples/loop_normalize_macro.hako ./target/release/hakorune --macro-preexpand --backend vm apps/tests/macro/loopform/simple.hako`
 
 Selfhost compiler prepass（恒等→最小正規化）
 - Runner が `NYASH_LOOPFORM_NORMALIZE=1` を `--loopform` にマップして子に渡し、`apps/lib/loopform_normalize.hako` の前処理を適用（現状は恒等）。