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>

plugins/nyash-array-plugin/src/lib.rs

@@ -103,8 +103,14 @@ pub extern "C" fn nyash_plugin_invoke(
                 if let Ok(mut map) = INSTANCES.lock() {
                     if let Some(inst) = map.get_mut(&instance_id) {
                         let i = idx as usize;
-                        if i >= inst.data.len() { return NYB_E_INVALID_ARGS; }
-                        inst.data[i] = val;
+                        let len = inst.data.len();
+                        if i < len {
+                            inst.data[i] = val;
+                        } else if i == len {
+                            inst.data.push(val);
+                        } else {
+                            return NYB_E_INVALID_ARGS;
+                        }
                         return write_tlv_i64(inst.data.len() as i64, result, result_len);
                     } else { return NYB_E_INVALID_HANDLE; }
                 } else { return NYB_E_PLUGIN_ERROR; }