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>

apps/tests/ny-map-llvm-smoke/main.nyash

@@ -0,0 +1,19 @@
+// ny-map-llvm-smoke - MapBox by-id plugin path smoke
+// 目的: MapBox(size/get/set) を by-id 経由で実行し代表出力を確認
+
+static box Main {
+    main() {
+        // Map を生成し、1->42 を設定
+        local m = new MapBox()
+        m.set(1, 42)
+        
+        // 取得とサイズ
+        local v = m.get(1)
+        local s = m.size()
+
+        // 代表出力（連結は1回ずつに限定してLLVM簡易対応）
+        print("Map: v=" + v)
+        print("size=" + s)
+        return 0
+    }
+}