Implements binary codec for meta-group snapshots: 2-byte little-endian version
header followed by S2-compressed JSON of the stream assignment map. Adds
[JsonObjectCreationHandling(Populate)] to StreamAssignment.Consumers so the
getter-only dictionary is populated in-place during deserialization. 8 tests
covering round-trip, compression ratio, field fidelity, multi-consumer restore,
version rejection, and truncation guard.
Go reference: jetstream_cluster.go:2075-2145 (encodeMetaSnapshot/decodeMetaSnapshot)
