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>

create_dummy_ffi_ps.ps1

@@ -0,0 +1,17 @@
+# Create dummy ffi.lib
+$dummyContent = @"
+// Dummy FFI functions for AOT-only build
+void ffi_call() {}
+void ffi_prep_cif() {}
+void ffi_prep_closure_loc() {}
+void ffi_type_void() {}
+"@
+
+# Write C file
+$dummyContent | Out-File -FilePath "C:\LLVM-18\lib\ffi_dummy.c" -Encoding UTF8
+
+Write-Host "Created dummy C file at C:\LLVM-18\lib\ffi_dummy.c"
+Write-Host "Now you need to compile it with Visual Studio Developer Command Prompt:"
+Write-Host "  cd C:\LLVM-18\lib"
+Write-Host "  cl /c ffi_dummy.c"
+Write-Host "  lib /OUT:ffi.lib ffi_dummy.obj"