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/10-Now.md

@@ -2,12 +2,9 @@
 
 ## Current Focus (next)
 
-- Phase 278（cleanup）: `docs/development/current/main/phases/phase-278/README.md`
-  - Phase 277 P2 の後方互換（旧PHI env var）を撤去して、1セットに収束させる
-- Phase 279（impl）: `docs/development/current/main/phases/phase-279/README.md`
-  - “2本のコンパイラ” にならないように、型伝播パイプラインの入口/順序を SSOT で一本化する
-- Phase 273（design-first）: `docs/development/current/main/30-Backlog.md`
-  - Pattern を Plan Extractor（pure）へ降格し、`Plan → Frag → emit_frag()` に収束させる
+- Phase 273（design-first）: `docs/development/current/main/phases/phase-273/README.md`
+  - P1: DomainPlan→CorePlan（固定語彙）+ PlanNormalizer（SSOT）+ PlanVerifier（fail-fast）で “増殖” を止める
+    - CorePlan は `Seq/Loop/If/Effect/Exit` のみ（scan専用 Effect/variant 禁止、式を String にしない）
 
 ## Recently Completed (2025-12-22)
 
@@ -15,6 +12,9 @@
 - Phase 276 P0（quick wins / type_helper SSOT）: `docs/development/current/main/phases/phase-276/README.md`
 - Phase 277 P1（PHI strict fail-fast）: `docs/development/current/main/phases/phase-277/README.md`
 - Phase 277 P2（PHI env var 統合）: `docs/development/current/main/phases/phase-277/README.md`
+- Phase 278 P0（deprecated PHI env vars removal）: `docs/development/current/main/phases/phase-278/README.md`
+- Phase 279 P0（type propagation pipeline SSOT unification）: `docs/development/current/main/phases/phase-279/README.md`
+- Phase 273 P0（Plan Extractor PoC）: `docs/development/current/main/phases/phase-273/README.md`
 
 ---