docs(edgecfg): Phase 280 - Frag Composition SSOT Positioning (A→B→C)

## Purpose

Stop pattern number enumeration proliferation by establishing Frag composition API
as the Single Source of Truth (SSOT) for structured control flow → CFG lowering.

Pattern numbers (1-9+) are **symptom labels** for regression tests, NOT architectural
concepts. The architectural SSOT is **Frag composition rules** (seq/if/loop/cleanup).

## Changes Summary

**Phase A (Docs-only, no code)**: SSOT Positioning
- edgecfg-fragments.md: Status Draft → Active SSOT (+243 lines)
  - Added 5 sections: Composition SSOT, Rules, Laws, Fail-Fast, Ownership
  - Documented 3-tier ownership model (Normalizer/Composition/Lowerer)
  - Established composition as pattern absorption destination
- joinir-architecture-overview.md: Pattern absorption documentation (+90 lines)
  - Added Section 0.2: Pattern Number Absorption Destination
  - JoinIR vs Plan comparison (different extraction, same SSOT)
  - Pattern absorption status table (Pattern6/7 as Phase 280 targets)
- phase-280/README.md: Full roadmap (new)

**Phase B (API solidification)**: Contract Verification
- compose.rs: Module-level + function-level Phase 280 docs (+149 lines)
  - Documented composition SSOT, ownership model, usage example
  - Added constraint/composition law sections to seq/if/loop/cleanup
  - Contract verification: All seq/if/loop contracts verified (no gaps)
  - Test gap analysis: No missing tests (wires/exits separation explicitly tested)

**Phase C (Pattern preparation)**: Documentation-only
- normalizer.rs: Pattern6/7 TODO comments (+10 lines)
  - Pattern6: Early exit doesn't fit compose::if_() → cleanup() target
  - Pattern7: 挙動不変保証難 → compose::if_() migration deferred to Phase 281

## Impact

- **Net +460 lines** (docs-heavy, minimal code)
- **4 files modified**, 1 directory created
- **SSOT established**: Frag composition is now THE absorption destination
- **導線固定**: Clear migration path for Pattern6/7 (Phase 281+)
- **No behavior change**: Documentation-only for Phase C (tests not run)

## Phase 280 Goal Achieved

✅ SSOT positioning + 導線固定 (NOT full migration - that's Phase 281)
✅ Phase A complete: Docs updated to "Active SSOT"
✅ Phase B complete: API contract verified and documented
✅ Phase C complete: Pattern6/7 hand-rolled locations documented

## Next Phase (Phase 281+)

- Phase 281: Full Pattern6/7 absorption (replace hand-rolled with compose_*)
- Phase 282: Router shrinkage (pattern numbers → test labels)
- Phase 283+: Pattern8 and beyond

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

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

docs/development/current/main/joinir-architecture-overview.md

@@ -136,6 +136,96 @@ NYASH_JOINIR_DEBUG=1 cargo test --release --lib
 実装: `src/config/env/joinir_flags.rs::is_joinir_debug()` が両者をチェック。
 詳細: `docs/development/current/main/phase82-83-debug-flag-ssot-summary.md`
 
+## 0.2 Pattern Number Absorption Destination (Phase 280)
+
+**Status**: Active (2025-12-23)
+**Purpose**: Stop pattern enumeration proliferation by establishing Frag composition as SSOT
+
+### The Problem: Pattern Enumeration Proliferation
+
+Pattern numbers (1-9+) became architectural decision points:
+- Router branches exploded (17+ patterns across JoinIR/Plan routes)
+- Each pattern duplicated CFG construction logic (block allocation, PHI insertion, terminator emission)
+- "Pattern-specific" knowledge leaked into lowering layers
+- Adding new loop shapes required full-stack pattern additions
+
+**Symptom**: Pattern numbers drove architecture instead of being test labels
+
+### The Solution: Frag Composition SSOT
+
+**Key Insight**: Pattern numbers → symptom labels (test names), CFG construction → Frag composition API
+
+**Architecture shift**:
+- **Before Phase 280**: Pattern number → entire lowering pipeline (extractor + allocator + emitter)
+- **After Phase 280**: Pattern number → extractor only, all lowering uses Frag composition SSOT
+
+**Composition API as absorption destination**:
+- `seq(a, b)`: Sequential composition (Normal wiring)
+- `if_(header, cond, t, e, join)`: Conditional composition (Branch wiring)
+- `loop_(loop_id, header, after, body)`: Loop composition (Break/Continue wiring)
+- `cleanup(body, cleanup)`: Cleanup composition (planned, Phase 280+)
+
+**Reference**: `docs/development/current/main/design/edgecfg-fragments.md` (Active SSOT)
+
+### JoinIR vs Plan: Different Extraction, Same SSOT
+
+Both routes converge on the same Frag composition SSOT:
+
+| Route | Extraction Source | Pattern Knowledge | Composition SSOT |
+|-------|-------------------|-------------------|------------------|
+| **JoinIR** | cf_loop structure (Structured JoinIR) | JoinIR-specific (cf_loop DSL) | **Frag API** (seq/if/loop) |
+| **Plan** | DomainPlan (Pattern6/7 extractors) | Domain-specific (ScanWithInit/SplitScan) | **Frag API** (same) |
+
+**Key principle**: Different extraction strategies, converged CFG construction
+
+**Why separate routes?**:
+- JoinIR route: Handles cf_loop-based patterns (Pattern1-5, 8-9) via Structured JoinIR
+- Plan route: Handles complex scan patterns (Pattern6/7) via DomainPlan → CorePlan → Frag
+- Both routes use same Frag composition API for CFG lowering (no duplication)
+
+### Pattern Absorption Status (Phase 280)
+
+| Pattern | Structure | Status | Absorption Target |
+|---------|-----------|--------|-------------------|
+| **Pattern6** | ScanWithInit (forward scan) | Plan-based (Phase 273) | **Phase 280 C: Frag refactor target** |
+| **Pattern7** | SplitScan (conditional split) | Plan-based (Phase 273) | **Phase 280 C: Frag refactor target** |
+| Pattern8 | BoolPredicateScan (is_integer) | JoinIR-based | Phase 281+ (migration planned) |
+| Pattern9 | AccumConstLoop (bridge) | JoinIR-based (Phase 271) | 撤去条件 defined (minimal loop SSOT) |
+| Pattern1-5 | Legacy (SimpleWhile, Break, IfPhi, Continue, InfiniteEarlyExit) | JoinIR-based | Test/error stubs (not absorbed) |
+
+**Phase 280 Focus**: Pattern6/7 preparation (documentation of hand-rolled locations, defer refactor to Phase 281)
+
+**Absorption criteria** (Pattern6/7 → Frag composition):
+1. Hand-rolled Frag construction identified (function name + 識別コメント)
+2. TODO comments documenting future compose_* migration path
+3. Behavior-preserving refactor deferred to Phase 281 (Phase 280 = SSOT positioning only)
+
+### Absorption Timeline
+
+**Phase 280 (Current)**: SSOT positioning + 導線固定
+- A: Documentation (edgecfg-fragments.md → Active SSOT)
+- B: API solidification (compose.rs contract verification)
+- C: Pattern preparation (Pattern6/7 hand-rolled locations documented)
+- **Goal**: Establish Frag composition as THE absorption destination
+- **Non-Goal**: Full Pattern6/7 migration (deferred to Phase 281)
+
+**Phase 281 (Planned)**: Full Pattern6/7 absorption
+- Replace hand-rolled Frag construction with compose_* calls
+- Behavior-preserving verification (smoke tests)
+- Goal: Pattern6/7 use compose::if_/loop_ exclusively
+
+**Phase 282 (Planned)**: Router shrinkage
+- Pattern numbers → test labels only
+- Router uses Frag composition for all CFG construction
+- Pattern extractors remain as thin detection layer
+
+**Phase 283+ (Future)**: Pattern8 and beyond
+- Migrate Pattern8 to Plan route or Frag-based JoinIR
+- Evaluate Pattern9 撤去 (if minimal loop SSOT achieved)
+- Continue pattern number reduction
+
+---
+
 ## 1. 不変条件（Invariants）
 
 JoinIR ラインで守るべきルールを先に書いておくよ：