feat(phase21.5): Loop FORCE direct assembly + PHI/compare fixes

## Loop FORCE Direct Assembly ✅
- Added: Direct MIR assembly bypass when HAKO_MIR_BUILDER_LOOP_FORCE_JSONFRAG=1
- Implementation: Extracts limit from Program(JSON), generates minimal while-form
- Structure: entry(0) → loop(1) → body(2) → exit(3)
- PHI: i = {i0, entry} | {i_next, body}
- Location: tools/hakorune_emit_mir.sh:70-126
- Tag: [selfhost-direct:ok] Direct MIR assembly (FORCE=1)

## PHI/Compare Fixes (ny-llvmc) ✅
- Fixed: vmap maintenance for PHI results across instructions
- Fixed: PHI placeholder name consistency (bytes vs str)
- Fixed: ensure_phi_alloca creates unique placeholders per block
- Fixed: resolve_i64_strict properly looks up PHI results
- Files:
  - src/llvm_py/phi_wiring/tagging.py
  - src/llvm_py/phi_wiring/wiring.py
  - src/llvm_py/instructions/compare.py
  - src/llvm_py/resolver.py

## Testing Results
- VM backend: ✅ rc=10 (correct)
- Direct assembly MIR: ✅ Structurally correct
- Crate backend: ⚠️ PHI/compare issues (being investigated)

## Implementation Principles
- 既定挙動不変 (FORCE=1 gated)
- Dev toggle controlled
- Minimal diff, surgical changes
- Bypasses using resolution when FORCE=1

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

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

tools/ny_mir_builder.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 # ny_mir_builder.sh — Minimal MIR Builder CLI (shell wrapper)
-# Purpose: consume Nyash JSON IR and emit {obj|exe|ll|json} using the existing nyash LLVM harness.
+# Purpose: consume Nyash JSON IR and emit {obj|exe|ll|json} via the ny-llvmc crate backend by default.
+# Notes: llvmlite harness remains internal; choose it explicitly with NYASH_LLVM_BACKEND=llvmlite when debugging.
 
 set -euo pipefail
 [[ "${NYASH_CLI_VERBOSE:-0}" == "1" ]] && set -x