byte-exact reader for upstream's sealed segment format (the wire-visible archive contract served raw by getSegment/getBlock): 256B header with xxh3 verification (header[12..]||footer), 52B block-index entries, zstd frame walk, columnar block decode (9 fixed columns + 5 blobs, exact-length enforcement, empty compaction blocks), collection block index (zstd body, table + per-block bitmasks). blooms skipped (query accelerators; no-false-negative contract means correct iteration doesn't need them). docs/jss-format-v1.md carries the full byte layout distilled from upstream source. differential fixtures: a sealed segment produced by upstream jetstream bootstrapping 50 simulator repos + its inspect-segment output, committed under tests/fixtures/. tests assert our parse matches their inspection field-for-field (checksum, per-block stats, collection counts), that all 7018 events decode in seq order, and that every commit payload is intact DAG-CBOR. known gap: zstd frame CRC not verified (unimplemented in zig std, panics if enabled) — the file-level xxh3 covers header+footer only. revisit when the writer lands with libzstd. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>