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MsgBlock is the unit of storage in the file store — a single append-only block file containing sequentially written binary message records. Blocks are sealed (read-only) when they reach a configurable byte-size limit. Key features: - Write: appends MessageRecord-encoded messages with auto-incrementing sequence numbers and configurable first sequence offset - Read: positional I/O via RandomAccess.Read for concurrent reader safety - Delete: soft-delete with on-disk persistence (re-encodes flags byte + checksum so deletions survive recovery) - Recovery: rebuilds in-memory index by scanning block file using MessageRecord.MeasureRecord for record boundary detection - Thread safety: ReaderWriterLockSlim allows concurrent reads during writes Also adds MessageRecord.MeasureRecord() — computes a record's byte length by parsing varint field headers without full decode, needed for sequential record scanning during block recovery. Reference: golang/nats-server/server/filestore.go:217-267 (msgBlock struct) 12 tests covering write, read, delete, seal, recovery, concurrency, and custom sequence offsets.