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/mir-nested-branch/main.nyash

@@ -0,0 +1,20 @@
+// mir-nested-branch - nested branches with returns
+
+static box Main {
+    main() {
+        if 1 < 2 {
+            if 3 < 4 {
+                return 1
+            } else {
+                return 2
+            }
+        } else {
+            if 5 < 6 {
+                return 3
+            } else {
+                return 4
+            }
+        }
+    }
+}
+