HA: the late-sample record of a continuous aggregate is shipped right after the append commits, so a leader lost in between drops it #10673

Follow-up of #10634 (derived from reading the code, not reproduced on a cluster).

Since #10634 a sample appended behind the watermark of a continuous aggregate ships its late-sample record (#10502) to the followers in a schema recording session of its own (TimeSeriesShard.invalidateContinuousAggregatesBehindWatermark -> LocalSchema.recordContinuousAggregateChange). That session runs right after the append's own transaction committed, not inside it: the append commits through a nested, self-owned transaction under the shard's append lock (#7410/#7657), and the record must not be written before the commit (a refresh reading the source before it would clear it).

Remaining gap: a leader lost between the append's TX_ENTRY and the schema entry carrying the record leaves the followers with the samples and without the record. A follower promoted then never aggregates those late samples into their buckets (until a delete or a TRUNCATE invalidates them again).

Other related, non-gaps (for the record):

• The refresh's own state (watermark advance, clearing of the record) is still saved with a plain schema save: the refresh runs from after-commit callbacks, under the database read lock, where a schema session cannot be opened. An unset watermark beside existing backing rows is now reconciled from the backing type at the next refresh (HA: the late-sample record and the watermark of a continuous aggregate are saved outside a schema session, so a promoted follower refreshes without them #10634); a stale set watermark or a stale record only causes extra recompute, and every compaction/retention session ships the leader's current schema.

Options: carry the record inside the append's replication unit (e.g. an optional section of the TX_ENTRY, which needs a capability per ha-raft/CLAUDE.md), or have a newly elected leader treat the buckets of the samples appended in the last N entries as invalidated.