Per-corpus shard: changes-feed subscriber, segment writer, and compaction coordinator.

The shard keeps a corpus in sync with its barrel_docdb database. It subscribes to the changes feed in push mode and applies each batch to an in-memory buffer keyed by document id (an update replaces, a delete becomes a tombstone). The buffer holds each live key's corpus TEXT, not pre-computed grams: gram selection (both phase-1 dense and phase-2 positional) happens once, at freeze time, from that text, rather than once per change -- a document updated several times before a freeze only ever has its final version's grams computed once, and the buffer is not searched directly (every buffered key is always a candidate), so nothing needs its grams before freeze. When the buffer passes a threshold it freezes to a new immutable segment and commits the manifest, advancing the persisted watermark.

Segments only ever accumulate on their own, so when the live count crosses a threshold the shard compacts: an offloaded worker merges the segments, collapsing each key to its newest version by HLC and dropping superseded and deleted ordinals, and the shard swaps the manifest to the merged segment. compact/1 does this synchronously.

Recovery is the watermark: on start the shard loads the manifest and resubscribes from its watermark, so only the feed tail is replayed (idempotently). Correctness of updates/deletes never depends on compaction, the query confirm pass re-fetches the current document and drops a stale or deleted candidate.