feat: wire dispatch registry, replace type-switch, add MockBackend

- Fill register_stubs.go with 115 handler wrappers for all AST types
- Replace 308-line type-switch in executor_dispatch.go with registry.Dispatch()
- Replace goroutine+time.After with context.WithTimeout in Execute()
- Add MockBackend (function-field pattern) in mdl/backend/mock/
- Add registry completeness test verifying all 115 statement types
- Fix pre-existing bug: ShowFeaturesStmt was missing from dispatch

Author: retran