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-278/README.md

@@ -1,6 +1,6 @@
-# Phase 278 (planned): Remove deprecated PHI debug env vars
+# Phase 278 P0: Remove deprecated PHI debug env vars
 
-Status: planned / cleanup
+Status: ✅ completed (2025-12-22)
 
 Goal: remove legacy PHI debug environment variables that were consolidated in Phase 277 P2, so the ecosystem converges to a single, memorable set.
 
@@ -16,3 +16,5 @@ Target SSOT (post-Phase 278):
 Implementation guide:
 - `docs/development/current/main/phases/phase-278/P0-INSTRUCTIONS.md`
 
+Completion:
+- `docs/development/current/main/phases/phase-278/P0-COMPLETION.md`