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>

build_llvm_wsl.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e
+
+echo "Building Nyash with LLVM for Windows from WSL..."
+
+# Windows側のLLVMを使う
+export LLVM_SYS_180_PREFIX="/mnt/c/Program Files/LLVM"
+
+# 追加の環境変数（Qt6ビルドで使っていたかもしれない技）
+export LLVM_SYS_180_FFI_WORKAROUND=1
+export CC=x86_64-w64-mingw32-gcc
+export CXX=x86_64-w64-mingw32-g++
+export AR=x86_64-w64-mingw32-ar
+
+# MinGWターゲットで試す（Qt6と同じ方法）
+echo "Trying MinGW target..."
+cargo build --target x86_64-pc-windows-gnu --release --features llvm
+
+# 成功したら実行ファイルの場所を表示
+if [ $? -eq 0 ]; then
+    echo "Build successful!"
+    echo "Binary at: target/x86_64-pc-windows-gnu/release/nyash.exe"
+else
+    echo "MinGW build failed, trying MSVC target with cargo-xwin..."
+    cargo xwin build --target x86_64-pc-windows-msvc --release --features llvm
+fi