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>

src/mir/builder/control_flow/plan/normalizer.rs

@@ -295,6 +295,10 @@ impl PlanNormalizer {
             values: vec![i_current],
         };
 
+        // Phase 280 TODO: Hand-rolled Frag construction for early exit pattern
+        // Reason: `found_bb` is early Return, doesn't fit compose::if_() model
+        // Future: Consider compose::cleanup() for early exit normalization (Phase 281+)
+        // Current structure: 2 BranchStub (header→body/after, body→found/step) + 2 EdgeStub (step→header, found→Return)
         let branches = vec![
             BranchStub {
                 from: header_bb,
@@ -688,6 +692,12 @@ impl PlanNormalizer {
         ];
 
         // Step 12: Build Frag (2 branches + 3 wires)
+        //
+        // Phase 280 TODO: Hand-rolled Frag construction for split scan pattern
+        // Target (Phase 281): compose::if_(body_bb, cond_match, then_frag, else_frag, step_frag)
+        // Reason deferred: 挙動不変保証が難しい、Phase 280 は SSOT positioning 優先
+        // Migration: Phase 281+ で compose::if_() への移行を検討
+        // Current structure: 2 BranchStub (header→body/after, body→then/else) + 3 EdgeStub (then→step, else→step, step→header)
         let empty_args = EdgeArgs {
             layout: JumpArgsLayout::CarriersOnly,
             values: vec![],