moq-lite-05: add Frame Start to FETCH

[kixelated]

What

Implements moq-dev/drafts#24. Adds a Frame Start field to the moq-lite FETCH message in both the Rust (rs/moq-net) and JS (js/net) implementations.

FETCH Message {
  ...
  Group Sequence (i)
  Frame Start (i)   <-- new (lite-05+)
}

Why

Today a FETCH always returns a group from frame 0. Frame Start is the 0-based index of the first frame to return; the publisher skips all earlier frames, letting a subscriber resume partway through a partially-received group. 0 returns the entire group.

Details

• Rust (rs/moq-net/src/lite/fetch.rs): added frame_start: u64 to Fetch, encoded/decoded only on Lite05Wip+. Lite03/Lite04 omit it and default to 0, following the existing version-gating convention (older drafts listed explicitly, newest as the _ fallthrough).
• JS (js/net/src/lite/fetch.ts): added frameStart (default 0) with matching version-gated encode/decode; threaded version into the private encode/decode helpers.
• Tests added on both sides: round-trip on draft-05 and "absent before draft-05" (verifying the older encoding is strictly shorter).

Scope notes

• The Start Group/End Group → Group Start/Group End rename also in drafts#24 is, per its own changelog entry, wire format unchanged (spec prose only). The implementations already match the wire, so internal field names were left as-is to avoid a large, behavior-neutral churn across public moq-net APIs and downstream consumers.
• FETCH is currently a wire definition only; it is not yet plumbed into a live serving flow, so there is no publisher-side skip/wait behavior to implement here. The field is in place for when that flow lands.

Test plan

• cargo test -p moq-net lite::fetch
• bun test js/net/src/lite/fetch.test.ts
• cargo fmt --check + Biome clean

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

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(Written by Claude)