v0.20.0

What's new

• Cascade destroy: DestroyVariable removes variable and all descendants, notifies per-variable (children before parents)
• Resolver fix: prevent double-processing in ConvertToValueJSON — array elements left as raw Go values for caller
• ViewList fix: use variable.Value instead of NavigationValue() during init
• Frontend: clean up errors map on variable destroy
• Architecture guide: new UNDERSTANDING-UI-ENGINE.md explaining the inverted-control model
• Exports: expose ExpandVerbosityFlags, BundleCreateBundle, BundleExtractBundle
• Dependency: bump BurntSushi/toml v1.4.0 → v1.5.0