One codec, `packages/cultmesh-ts/src/realtime-wire.ts`, rule for rule what
`CultMeshRealtimeWireProtocol` does: the 37-byte little-endian fixed header
with magic 0x31545343, the delivery byte as the C# enum ordinal, the i64
epoch and sequence, three u16 identity lengths, the i32 payload length, the
stamped header size and wire version, then UTF-8 identities and the payload.
It touches no socket; QUIC stream framing stays transport-owned. The blank
reading in `Validate` is `IsNullOrWhiteSpace`, whose whitespace set is not
`String.prototype.trim`'s, so it borrows `isNullOrWhiteSpaceCSharp` from
`cultnet-ts` rather than growing a second reading of the same rule.
Byte parity is proven in both directions and in two processes, which is what
the authority vectors cost us to learn. `contracts/cultmesh/realtime-frame-vectors.json`
is written by the new C# test under CULTMESH_WRITE_VECTORS=1 and decoded and
re-encoded here; `realtime-frame-vectors.ts-written.json` is written by
`scripts/write-cultmesh-realtime-frame-vectors.mjs` with this codec, over
different identities and epochs, and decoded by the same C# test with the
reference implementation alone. Both files share one field shape, so each
side reads both with one reader. The two cases that would put megabytes into
a committed contract file, identities at exactly 65,535 UTF-8 bytes and a
64 MiB payload, are recorded as a build rule plus the digest of the whole
encoding; both sides build them the same way.
Three things the C# reference gets from its type system and TypeScript does
not, each refused rather than left to surface as a stray runtime error: a
`bigint` outside the i64 range, a delivery string the union cannot spell, and
a leading U+FEFF, which `TextDecoder` deletes by default and
`Encoding.UTF8.GetString` keeps. The last one is a real divergence the test
caught; the decoder now sets `ignoreBOM`. U+FEFF is not whitespace to .NET,
so such an identity passes the transport contract and reaches the wire.
There was no C# test for this codec before; this is also its first.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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