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>

src/cli/args.rs

@@ -55,7 +55,7 @@ pub fn build_command() -> Command {
         .arg(Arg::new("hako-run").long("hako-run").help("Run via Stage-1 (.hako) stub (equivalent to NYASH_USE_STAGE1_CLI=1)").action(clap::ArgAction::SetTrue))
         .arg(Arg::new("program-json-to-mir").long("program-json-to-mir").value_name("FILE").help("Convert Program(JSON v0) to MIR(JSON) and exit (use with --json-file)"))
         .arg(Arg::new("emit-exe").long("emit-exe").value_name("FILE").help("Emit native executable via ny-llvmc and exit"))
-        .arg(Arg::new("emit-exe-nyrt").long("emit-exe-nyrt").value_name("DIR").help("Directory containing libnyash_kernel.a (used with --emit-exe)"))
+        .arg(Arg::new("emit-exe-nyrt").long("emit-exe-nyrt").value_name("DIR").help("Directory containing libnyash_kernel.a (used with --emit-exe). Hint: build via `cargo build -p nyash_kernel --release` (default output: target/release/libnyash_kernel.a)"))
         .arg(Arg::new("emit-exe-libs").long("emit-exe-libs").value_name("FLAGS").help("Extra linker flags for ny-llvmc when emitting executable"))
         .arg(Arg::new("stage3").long("stage3").help("Enable Stage-3 syntax acceptance for selfhost parser").action(clap::ArgAction::SetTrue))
         .arg(Arg::new("ny-compiler-args").long("ny-compiler-args").value_name("ARGS").help("Pass additional args to selfhost child compiler"))