Server: a write with the current base token succeeds and returns the advanced
token; a stale base returns 409 and writes nothing (no unconditional fallback);
a malformed base 422s; an omitted base keeps the unconditional write. Client:
the ring payload never carries widgets and threads the base token, the shared
busy gate covers either write, and the Save/ring wiring is source-pinned.
Refs #581