feat(llvm/phi): Phase 277 P1 - fail-fast validation for PHI strict mode

## Summary
Implemented fail-fast validation for PHI ordering and value resolution in strict mode.

## Changes

### P1-1: Strict mode for "PHI after terminator"
- File: `src/llvm_py/phi_wiring/wiring.py::ensure_phi`
- Behavior: `NYASH_LLVM_PHI_STRICT=1` → RuntimeError if PHI created after terminator
- Default: Warning only (no regression)

### P1-2: Strict mode for "fallback 0"
- File: `src/llvm_py/phi_wiring/wiring.py::wire_incomings`
- Behavior: Strict mode forbids silent fallback to 0 (2 locations)
  - Location 1: Unresolvable incoming value
  - Location 2: Type coercion failure
- Error messages point to next debug file: `llvm_builder.py::_value_at_end_i64`

### P1-3: Connect verify_phi_ordering() to execution path
- File: `src/llvm_py/builders/function_lower.py`
- Behavior: Verify PHI ordering after all instructions emitted
- Debug mode: Shows "✅ All N blocks have correct PHI ordering"
- Strict mode: Raises RuntimeError with block list if violations found

## Testing
✅ Test 1: strict=OFF - passes without errors
✅ Test 2: strict=ON - passes without errors (no violations in test fixtures)
✅ Test 3: debug mode - verify_phi_ordering() connected and running

## Scope
- LLVM harness (Python) changes only
- No new environment variables (uses existing 3 from Phase 277 P2)
- No JoinIR/Rust changes (root fix is Phase 279)
- Default behavior unchanged (strict mode opt-in)

## Next Steps
- Phase 278: Remove deprecated env var support
- Phase 279: Root fix - unify "2本のコンパイラ" pipelines

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

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

docs/tools/cli-options.md

@@ -8,7 +8,7 @@
 - `--debug-fuel {N|unlimited}`: パーサーのデバッグ燃料（無限ループ対策）
 
 ## MIR関連
-- `--dump-mir`: MIRを出力（実行はしない）
+- `--dump-mir`: MIRを出力（実行はしない / compile-only。実行経路SSOTの確認は `NYASH_VM_DUMP_MIR=1 --backend vm` を優先）
 - `--verify`: MIR検証を実施
 - `--mir-verbose`: 詳細MIR出力（統計など）