quest: plan the release backport of #7135's joined-head FETCH race fix

quest/m0/README.md

@@ -26,6 +26,7 @@ edge, which embeds moq-relay; it moved there on 2026-09-28.
 - [Release shutdown backport](/quest/m0/ffi-shutdown-backport.md) - `release` publishes a non-breaking moq-ffi with an async `drain()` and qmux sessions on the QUIC stream limit
 - [Backport audio groupDuration](/quest/m0/backport-audio-group-duration.md) - `@moq/publish` 0.5.4 on `release` can pack several Opus frames per group, making Cloudflare's relay drop audio less often
 - [Backport the SETUP-path and iroh-key fixes](/quest/m0/backport-security-fixes.md) - `release` refuses a conflicting SETUP Path and keeps persisted iroh keys private, in one patch release
+- [Backport the joined-head FETCH race fix](/quest/m0/backport-fill-head-race.md) - a `release` relay stops FETCHing a group head its own SUBSCRIBE is bringing, which left live readers hanging
 - [CI cost and latency](/quest/m0/ci/README.md) - cloud CI runs only in a batched merge gate as one `just ci` pass per batch, expensive checks run nightly and gate `release`, caches move to R2, and runners are chosen last
 - [More tests under load](/quest/m0/test-flakes-2/README.md) - load-only and queue failures, one quest per flake, fixed at the cause, since each one fails a whole batch
 - [Backport the dot-segment and auth-redirect refusals](/quest/m0/backport-release-hardening.md) - `release`'s relay refuses dot-segment paths and its auth client refuses redirects

quest/m0/backport-fill-head-race.md

@@ -0,0 +1,40 @@
+# [M] Backport the joined-head FETCH race fix to release
+
+## Goal
+
+A relay on `release` no longer FETCHes the head of a group that its own
+SUBSCRIBE is already bringing. In that race the fetched objects can land first,
+so the relay caches the group as fetched, refuses the live tail stream that ends
+it as `duplicate`, and leaves live readers waiting on the group until they time
+out. This ports fix 2 of
+[moq-dev/moq#7135](https://github.com/moq-dev/moq/pull/7135) and ships it in a
+`moq-net` patch release.
+
+## Plan
+
+Decided 2026-10-30 (maintainer): port only the relay fix. Why: #7135's other
+fix, the JS publisher answering an `INCLUDE_PROPERTIES=0` FETCH without
+timestamps, cannot happen on `release`, whose `js/net/src/ietf/publisher.ts`
+`runFetch` refuses every FETCH.
+
+Decided 2026-10-30 (maintainer): m0, beside the other `release` backports.
+Why: it is an intermittent relay hang on the shipped line.
+
+`release` has the same machinery as main before #7135: the joining-head
+`Fill` in `rs/moq-net/src/ietf/subscriber.rs` and the group FETCH that fills a
+cache miss (`run_group_fetch`). It has no `rs/moq-net/src/model/track/fill.rs`,
+so the fix is hand-ported, not cherry-picked. Port the behavior: a cache miss
+for the joined group waits until the subscription's own head arrives or its
+request settles, then the cache answers it; a failed head request still falls
+back to a FETCH. Only that group waits, so older history still FETCHes at once,
+and the wait ends when no reader wants the group or the track aborts. Port
+#7135's subscriber tests that cover this alongside it.
+
+Start by reproducing the race on `release` with one of those tests. If
+`release` cannot hit it, delete this quest and say why in the PR.
+
+Public API: none. Wire: none.
+
+## Required
+
+- [The browser backend baseline completes on IETF22](/quest/m0/nightly/browser-baseline.md) - lands the fix this quest ports, on main