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/P0-COMPLETION.md

@@ -0,0 +1,34 @@
+# Phase 278 P0: Deprecated PHI env vars removal — completion
+
+Status: ✅ completed (2025-12-22)
+
+Goal:
+- Remove legacy PHI debug environment variables (previously consolidated in Phase 277 P2).
+- Enforce a single SSOT set of PHI debug knobs (fail-fast on deprecated inputs).
+
+SSOT (kept):
+- `NYASH_LLVM_DEBUG_PHI=1`
+- `NYASH_LLVM_DEBUG_PHI_TRACE=1`
+- `NYASH_LLVM_PHI_STRICT=1`
+
+Removed inputs (deprecated variables):
+- `NYASH_LLVM_PHI_DEBUG`
+- `NYASH_PHI_TYPE_DEBUG`
+- `NYASH_PHI_ORDERING_DEBUG`
+- `NYASH_LLVM_TRACE_PHI`
+- `NYASH_LLVM_VMAP_TRACE`
+
+Behavior:
+- If any removed variable is set, the harness errors with a replacement hint and exits non-zero.
+- No new environment variables introduced.
+
+Docs:
+- `docs/reference/environment-variables.md` updated with:
+  - removed variable list
+  - migration table (old → new)
+  - example error message
+
+Tests:
+- Added a dedicated smoke verifying:
+  - deprecated vars fail
+  - SSOT vars still work

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`