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>

Cargo.toml

@@ -156,7 +156,8 @@ egui_extras = { version = "0.29", features = ["image"], optional = true }
 image = { version = "0.25", features = ["png", "ico"], optional = true }
 
 # LLVM backend deps (optional)
-inkwell = { version = "0.5.0", features = ["llvm18-0"], optional = true }
+# Move to LLVM 18.1.x toolchains
+inkwell = { version = "0.6.0", features = ["llvm18-1"], optional = true }
 
 # Windows API
 [target.'cfg(windows)'.dependencies]