Remove serialized state infrastructure

The serialized state feature (LOAD_SERIALIZED_STATE action, SerializedStateV1
types, getSerializedState/getSerlializedState functions) was unused external
API surface. Removing it simplifies the codebase and eliminates the only
caller of ensureWindowsAreOnScreen() without a parentDomNode argument.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: captbaritone