[claude] FW Lite: client stamps un-pushed commits as synced, hiding sync gaps

[Claude-drafted]

SyncRepository.UpdateSyncDate stamps a SyncDate onto every local commit whose SyncDate is null — regardless of whether it actually reached the server (SyncRepository.cs:13-24):

UPDATE Commits SET metadata = json_set(metadata, '$.ExtraMetadata.SyncDate', <now>)
WHERE json_extract(Metadata, '$.ExtraMetadata.SyncDate') IS NULL;

So a commit that was never pushed (e.g. one silently dropped from the push set) is marked synced on the next sync: the pending-commits count drops to 0 and no UI or telemetry signal ever appears.

Impact: a real single-commit loss on project blj-flex — a stranded CreateExampleSentenceChange that left an orphaned jsonPatch:ExampleSentence edit, which throws NotSupportedException in SnapshotWorker and has wedged FwHeadless since 2026-07-10 — was invisible for weeks because of this. It will mask any future drop too.

Fix: only stamp SyncDate for commits confirmed present on the server (derive from the acknowledged push / the server's returned sync state), not "all local commits with null SyncDate".

Related: the drop itself comes from the lossy (ClientId → millisecond-timestamp) SyncState watermark — see sillsdev/harmony#79 (duplicated-ClientId variant of the same watermark weakness); server-side commit-PK hardening in #2365.

Start with SyncRepository.UpdateSyncDate in SyncRepository.cs:13-24, then trace the acknowledged push and the server's returned sync state. Confirm how an unpushed commit can remain identifiable after sync. Done means SyncDate is applied only to commits confirmed present on the server, while unpushed commits remain pending and visible.