Release v19.17.0

Fixed

• WaitForCompletion() no longer times out on observers whose tag, event source type, or event stream type filters exclude the appended events. Matching events in the same batch still must be handled before the wait completes. (#4198)

Also in this PR (after review)

• Failed partitions are reported only for observers whose filters match the appended events, including observers that are already caught up.
• Cached subscription matches are refreshed when an observer unsubscribes or changes its filters or event types.
• The .NET client now says explicitly when it sends the first sequence number (additive proto field 8), so a batch that starts at sequence 0 is handled correctly. Legacy clients, invalid ranges and ranges over 10,000 events fall back to waiting through the target.
• An explicit Default event source type filters the same way in the append queue, in handling and in the wait.
• Observers that are caught up and have no failed partitions no longer trigger subscription lookups on each poll.

Filter-aware waits apply only to clients that send the first sequence number. Today that is the .NET client; the documentation says so.

Known gaps are tracked separately: catch-up/replay reads ignore tag, source-type and stream-type filters, and batch-level filtering in Observer.Handling (#4329).