Enhance StreamSnapshotService with CreateTarSnapshotAsync / RestoreTarSnapshotAsync
methods that produce a Snappy-compressed TAR archive (stream.json + messages/*.json).
Add CreateTarSnapshotWithDeadlineAsync for deadline-bounded snapshots, and a
SnapshotRestoreResult record carrying stats. 10 new unit tests in
JetStream/Snapshots/StreamSnapshotTests.cs exercise the full create/restore
round-trip, compression format, empty-stream edge case, and deadline enforcement.
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