Phase 11-12: LLVM backend initial, semantics layer, plugin unification

Major changes:
- LLVM backend initial implementation (compiler.rs, llvm mode)
- Semantics layer integration in interpreter (operators.rs)
- Phase 12 plugin architecture revision (3-layer system)
- Builtin box removal preparation
- MIR instruction set documentation (26→Core-15 migration)
- Cross-backend testing infrastructure
- Await/nowait syntax support

New features:
- LLVM AOT compilation support (--backend llvm)
- Semantics layer for interpreter→VM flow
- Tri-backend smoke tests
- Plugin-only registry mode

Bug fixes:
- Interpreter plugin box arithmetic operations
- Branch test returns incorrect values

Documentation:
- Phase 12 README.md updated with new plugin architecture
- Removed obsolete NYIR proposals
- Added LLVM test programs documentation

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

docs/archive/proposals/nyir/README.md

@@ -0,0 +1,10 @@
+# NyIR ドキュメント索引
+
+- 仕様（骨子）: `spec.md`
+- フェーズ計画（Phase 9.10）: `../予定/native-plan/issues/phase_9_10_nyir_spec.md`
+- BIDサンプル: `bid_samples/`（console.yaml / canvas.yaml）
+- 計画全体（Copilot用フルテキスト 9.7以降）: `../予定/native-plan/copilot_issues.txt`
+- 関連: ABI/BIDドラフトは `../予定/native-plan/box_ffi_abi.md`
+
+補足:
+- NyIR Core（25命令）は凍結。拡張は NyIR-Ext（exceptions/concurrency/atomics）で段階導入。