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>

src/runtime/mod.rs

@@ -12,6 +12,8 @@ pub mod unified_registry;
 pub mod nyash_runtime;
 pub mod gc;
 pub mod scheduler;
+pub mod global_hooks;
+pub mod semantics;
 // pub mod plugin_box;  // legacy - 古いPluginBox
 // pub mod plugin_loader;  // legacy - Host VTable使用
 pub mod type_meta;