moq-lite-05: add Start Frame to FETCH

[kixelated]

What

Adds a Start Frame field to the FETCH message in draft-lcurley-moq-lite.md, targeting moq-lite-05.

FETCH Message {
  Message Length (i)
  Broadcast Path (s)
  Track Name (s)
  Subscriber Priority (8)
  Group Sequence (i)
  Start Frame (i)   <-- new
}

Why

Today a FETCH always returns a group from frame 0. Start Frame lets a subscriber begin partway through a group — e.g. to resume a partially-received group without re-downloading frames it already has.

Details

• Start Frame is the 0-based index of the first frame to return; the publisher skips all frames before it. 0 returns the entire group.
• If Start Frame is >= the number of frames in the group, the publisher returns no FRAME messages and FINs the stream once the group completes (no-op rather than an error).
• The returned FRAME messages are otherwise unchanged: the first returned frame's Timestamp Delta is still delta-encoded from 0 (its absolute timestamp), not from the skipped frame, so the existing FRAME encoding is unaffected.
• Updated the Fetch stream prose and added a moq-lite-05 changelog entry.

Notes for reviewer

• Named Start Frame to match the existing Start Group/End Group convention in SUBSCRIBE.
• No End Frame bound was added — FETCH is single-group and runs to the end of the group. Happy to add a frame-count limit if wanted.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 89c8b007-8d70-483e-821d-f140175ec5bb

📥 Commits

Reviewing files that changed from the base of the PR and between a0a9586 and 7a904eb.

📒 Files selected for processing (1)

• draft-lcurley-moq-lite.md

Walkthrough

This PR extends the MOQ-Lite FETCH protocol message to support partial group resumption. A new Start Frame (i) field enables subscribers to request frame delivery starting at a specific index within a group rather than always from frame 0. The change includes the wire format field definition, normative semantics describing publisher behavior when skipping frames and handling out-of-range indices, and a changelog entry documenting the feature addition.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: adding a Start Frame field to the FETCH message in moq-lite-05.
Description check	✅ Passed	The description is well-related to the changeset, providing clear context on what the change does, why it's needed, implementation details, and notes for reviewers.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@draft-lcurley-moq-lite.md`:
- Around line 797-798: Replace the ambiguous phrase "currently in the group"
with an explicit timing rule: state that the comparison "Start Frame >= number
of frames in the group" is evaluated against the group's final frame count when
the group is closed (i.e., the publisher must defer the decision for open groups
until group close); update the sentence about returning no FRAME messages/FINing
the stream to say this happens only if Start Frame is greater than or equal to
the final frame count at group close, and ensure "Start Frame", "group", and
"FRAME messages" are referenced to locate the change.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: a9474d12-be2f-4d94-bfba-9c8efca82f14

📥 Commits

Reviewing files that changed from the base of the PR and between 5510ab2 and a0a9586.

📒 Files selected for processing (1)

• draft-lcurley-moq-lite.md