feat(moq-net): encode Hop ID as fixed-width 64-bit on lite-05

[kixelated]

Implements moq-dev/drafts#33 ("Encode Hop ID as fixed-width 64-bit integer") for moq-lite-05.

Hop IDs are randomly assigned, so a variable-length integer would almost never be shorter than its fixed-width equivalent. Switching to a fixed-width big-endian 64-bit integer also recovers the 2 bits a 62-bit varint would have cost (the value can now use the full 64-bit space).

Summary

Applies to the three moq-lite wire fields that carry Hop ID values, gated to lite-05+ so lite-04 and earlier keep the varint:

• ANNOUNCE_OK Hop ID → (64)
• ANNOUNCE Hop ID list → (64)
• ANNOUNCE_INTEREST Exclude Hop → (64)

Rust (rs/moq-net)

• New Version::hops_fixed_width() predicate. Older versions are listed explicitly so future versions default forward (per the version-matching convention).
• Origin's Encode/Decode are now concrete over lite::Version so they can branch: the fixed-width path uses big-endian put_u64/get_u64 and carries the full 64-bit space (no 62-bit cap); older versions keep the varint + cap.
• exclude_hop routes through Origin to share the same branch.

JS mirror (js/net)

• Added Reader.u64() / Writer.u64() fixed-width helpers (+ setBigUint64).
• hopsFixedWidth(version) predicate mirroring Rust.
• OriginSchema widened 62 → 64 bits. randomOrigin deliberately stays masked to 62 bits, because the same id must still varint-encode if a session negotiates lite-04.

Not included

The moqt relay-hops half of the draft PR (HOP_PATH list of 8-byte entries, EXCLUDE_HOP type 0x40B58 → 0x40B59 length-prefixed) targets the moq-transport KVP parameters, which are not implemented anywhere in this repo yet, so there is nothing to change.

Branch targeting

Targets dev per the branch-targeting rules (wire-protocol change under rs/moq-net).

Test plan

• cargo test -p moq-net (389 pass), incl. new tests: full u64::MAX origin round-trip, fixed-width byte count, exclude_hop round-trip across [0, 7, 1<<53, u64::MAX].
• Full Rust workspace builds (moq-relay / moq-bench included).
• bun test in js/net (180 pass), incl. new u64 fixed-width stream test; integration test exercises the AnnounceOk + hops flow end-to-end.
• clippy + cargo fmt --check (via nix), biome, and tsc --noEmit all clean.

(Written by Claude)

[kixelated]

This should be impossible?

[kixelated]

Right, that was dead code. A lite Version varint decodes via decode_quic, which masks the top 2 bits off, so the value already caps at 2^62-1 and the check could never fire. Dropped it in 562a96f. (Written by Claude)