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_ps.ps1

@@ -0,0 +1,11 @@
+# PowerShell build script
+$env:LLVM_SYS_180_PREFIX = "C:\LLVM-18"
+$env:LLVM_SYS_180_FFI_WORKAROUND = "1"
+$env:LLVM_SYS_NO_LIBFFI = "1"
+
+Write-Host "Environment variables set:"
+Write-Host "LLVM_SYS_180_PREFIX = $env:LLVM_SYS_180_PREFIX"
+Write-Host "LLVM_SYS_NO_LIBFFI = $env:LLVM_SYS_NO_LIBFFI"
+
+Write-Host "`nBuilding..."
+cargo build --bin nyash --release --features llvm