quest: keep the old tail on both 2022-7 legs across a restart

quest/m2/README.md

@@ -134,6 +134,7 @@ ranked just below deterministic TS legs, which it requires.
 - [Drop announce="never"](/quest/m2/publish-announce-never.md) - `@moq/publish` offers only announce modes a watcher can reach
 - [Deterministic H.265 TS legs](/quest/m2/ts-hitless-h265.md) - H.265's buffering period gets the same group anchor as H.264
 - [TS over RTP legs](/quest/m2/ts-rtp-egress.md) - two legs send identical RTP packets, headers included, so an ST 2022-7 receiver can merge them
+- [Old tail across a restart](/quest/m2/ts-sync-restart-tail.md) - both `--sync` legs send the old timeline's tail up to a restart, so an ST 2022-7 merge has no hole there
 - [moqsrc A/V sync](/quest/m2/moqsrc-av-sync.md) - a nightly test measures A/V skew from `moqsrc` through clock-synced sinks
 - [MP4 export](/quest/m2/mp4-export.md) - `moq export mp4 --output` records crash-safe fragments, then finishes a regular MP4 with moov at the end
 - [fMP4 edit lists](/quest/m2/fmp4-edit-lists.md) - the fMP4 importer applies edit lists to frame timestamps

quest/m2/ts-sync-restart-tail.md

@@ -0,0 +1,33 @@
+# [M] Keep the old tail on both 2022-7 legs across a restart
+
+## Goal
+
+Two `moq export ts --sync` legs that see a timeline restart (a marker or
+source restart that opens a new generation) both send the old timeline's
+tail up to the restart, so an ST 2022-7 receiver merging them has no hole
+there. Ordinary skips already cost nothing, because the other leg's copies
+fill in.
+
+## Plan
+
+Found in [moq-dev/moq#6304](https://github.com/moq-dev/moq/pull/6304)
+(RTP egress), which this builds on. Each leg's lane forgets the positions it
+already holds: `Lane::reset` and `Lane::jump` in
+`rs/moq-mux/src/container/ts/lane.rs` clear `from` when the restart is read.
+The old tail is laid about one delay later, so all of it is marked unsure and
+withheld, costing roughly 0.5 to 1 s of output on both legs at a video and
+audio restart.
+
+- Old-timeline lane knowledge stays valid until the rewind that adopts the
+  new generation.
+- Keeping `[from, end)` known across a skip jump or a partial group is sound.
+  A restart is not obviously sound: `reset()` sets `end` to 0, and
+  new-timeline positions can land inside the old range when a restart opens
+  no new generation. Settle that first: either every restart opens a
+  generation, or lane knowledge is kept per timeline.
+- Reproduce with a mocked-time two-leg test of a video and audio restart that
+  asserts the merged legs cover the old tail. Extend
+  `an_rtp_leg_sends_its_paced_tail_before_a_restart` in
+  `export_timing_test.rs` from that PR if it has merged.
+
+Public API: none.