docs(plan): Phase 273 P4 - Plan Line SSOT Documentation Finalization

Phase 273 P4 では、Plan ライン（Extractor → Normalizer → Verifier → Lowerer）を
"current operational SSOT" として文書化し、アーキテクチャの収束を明文化した。

## Changes

### router.rs docstring 更新
- "Phase 273 P3: Plan Line is Current SSOT for Pattern6/7" セクション追加
- ルーティング戦略を明示（Plan entry points → legacy table）
- SSOT Entry Points を列挙（Pattern6/7 Normalizer, Pattern1-5 各 Lowerer）

### phase-273/README.md 更新
- P3 completion section 追加（generalized CoreLoopPlan 移行完了）
- P3+ Legacy Removal section 追加（~174 lines 削除記録）
- P4 Proposal section 追加（Documentation Finalization チェックリスト）
- SSOT Documentation Entry Points リスト追加（5 つの SSOT 入口）

### joinir-architecture-overview.md 更新
- Section 2.1.2 "Plan-Based Patterns (Pattern6-7, Phase 273 P3)" 追加
- Plan Extractor, Normalizer, Verifier, Lowerer の Box 構造を文書化
- Plan line vs JoinIR line 比較表追加（収束性・SSOT 特性の対比）
- SSOT characteristics リスト追加（Normalizer SSOT, emit_frag SSOT 等）

## SSOT Entry Points（Phase 273 P3 完了時点）

1. **ルーティング**: `router.rs::route_loop_pattern()` - Pattern6/7 Plan entry points
2. **型定義**: `plan/mod.rs` - DomainPlan/CorePlan 固定語彙
3. **正規化**: `plan/normalizer.rs` - Pattern 固有知識一元管理
4. **検証**: `plan/verifier.rs` - fail-fast 不変条件（V2-V9）
5. **降格**: `plan/lowerer.rs` - Pattern-agnostic MIR emission

## Test

- ✅ VM regression: phase254_p0_index_of_vm.sh (PASS)
- ✅ LLVM regression: phase258_p0_index_of_string_llvm_exe.sh (PASS)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/router.rs

@@ -278,13 +278,27 @@ pub(crate) static LOOP_PATTERNS: &[LoopPatternEntry] = &[
 /// - No redundant pattern detection in detect functions
 /// - All patterns use structure-based classification
 ///
-/// # Phase 273 P1: DomainPlan → Normalizer → Verifier → Lowerer
+/// # Phase 273 P3: Plan Line is Current SSOT for Pattern6/7
 ///
-/// Pattern6 uses the new two-layer Plan architecture:
-/// - extract_scan_with_init_plan() → DomainPlan (pure extraction)
-/// - PlanNormalizer::normalize() → CorePlan (pattern knowledge expansion)
+/// This function implements the following routing strategy:
+/// 1. Try Plan-based Pattern6 (extract_scan_with_init_plan) → DomainPlan
+/// 2. Try Plan-based Pattern7 (extract_split_scan_plan) → DomainPlan
+/// 3. Fall through to legacy Pattern1-5 table for other patterns
+///
+/// The Plan line (Extractor → Normalizer → Verifier → Lowerer) is the
+/// current operational SSOT for Pattern6/7. Legacy patterns (1-5) use
+/// the traditional LoopPatternContext-based routing.
+///
+/// Plan-based architecture (Phase 273 P1-P3):
+/// - extract_*_plan() → DomainPlan (pure extraction, no builder)
+/// - PlanNormalizer::normalize() → CorePlan (pattern knowledge expansion, SSOT)
 /// - PlanVerifier::verify() → fail-fast validation
-/// - PlanLowerer::lower() → MIR emission (pattern-agnostic)
+/// - PlanLowerer::lower() → MIR emission (pattern-agnostic, emit_frag SSOT)
+///
+/// SSOT Entry Points:
+/// - Pattern6: src/mir/builder/control_flow/plan/normalizer.rs (ScanWithInit normalization)
+/// - Pattern7: src/mir/builder/control_flow/plan/normalizer.rs (SplitScan normalization)
+/// - Pattern1-5: src/mir/builder/control_flow/joinir/patterns/pattern*.rs (direct lowering)
 pub(crate) fn route_loop_pattern(
     builder: &mut MirBuilder,
     ctx: &LoopPatternContext,