feat(mir/llvm): Phase 273 P0-P1 DomainPlan→CorePlan + LLVM arg fix

Phase 273 P0-P1: Two-layer plan architecture
- DomainPlan: Pattern-specific knowledge (ScanWithInit)
- CorePlan: Fixed vocabulary (Seq, Loop, If, Effect, Exit)
- ValueId references only (String expressions forbidden)
- Pipeline: Extractor→Normalizer→Verifier→Lowerer

New plan/ module:
- mod.rs: Type definitions, SSOT spec
- normalizer.rs: DomainPlan→CorePlan + ID allocation
- verifier.rs: V1-V6 invariant checks (fail-fast)
- lowerer.rs: CorePlan→MIR (pattern-agnostic)

LLVM fix (ChatGPT):
- function_lower.py: Fix argument reference bug
- Phase 258 index_of_string now PASS on LLVM backend

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

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

docs/development/current/main/phases/phase-273/P0-COMPLETION.md

@@ -0,0 +1,34 @@
+# Phase 273 P0: Plan Extractor (pure) + PlanLowerer SSOT — completion
+
+Status: ✅ completed (2025-12-22)
+
+Goal:
+- Pattern を “Plan抽出プラグイン” に降格し、CFG/PHI/block/value 生成の責務を PlanLowerer に集約して、裾広がりを止める。
+
+What changed (P0):
+- Plan 語彙を最小導入（P0 は PoC として pattern-specific を 1 個だけ許容）
+- Extractor は pure（builder を触らない）
+- Lowerer が block/value/phi を作る唯一の場所
+- terminator SSOT（`Frag → emit_frag()`）は維持
+
+PoC target:
+- Pattern6（scan with init）
+
+Behavior / limits:
+- `dynamic_needle=true` は当時 `Ok(None)` で fallthrough（P1+）
+- `step_lit != 1`（reverse scan）も `Ok(None)` で fallthrough（P1+）
+
+Regression:
+- `tools/smokes/v2/profiles/integration/apps/phase254_p0_index_of_vm.sh` PASS
+
+Files:
+- `src/mir/builder/control_flow/plan/mod.rs`
+- `src/mir/builder/control_flow/plan/lowerer.rs`
+- `src/mir/builder/control_flow/joinir/patterns/pattern6_scan_with_init.rs`
+- `src/mir/builder/control_flow/joinir/patterns/router.rs`
+
+Next (P1+):
+- Plan語彙を固定（`Seq/Loop/If/Effect/Exit`）へ畳み、pattern-specific Plan の増殖を止める。
+
+Note (follow-up):
+- その後の修正で `dynamic_needle`（Phase 258 の string index_of 系）も PlanLowerer 側で handling され、JoinIR freeze 下でも fallthrough しない経路に寄せている（詳細は Phase 273 README を参照）。