fetch_weight_salt still documented a live/public-top fallback on a fetch miss,
but the weight setter removed that path: on a miss it retains its last-good
on-chain weights and burns to UID 0 only after a full epoch of misses. Update
the WeightSalt dataclass docstring, the fetch_weight_salt docstring, and the
warning log to describe the actual retain/burn behaviour so the "no live
fallback" invariant isn't misread from the comments. No behaviour change.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>