quest: plan playout's move to positioned bus channels

quest/m0/README.md

@@ -106,6 +106,8 @@ repository.
 - [moq-video's browser backends are pinned](/quest/m0/pin-browser-video.md) - the web studio's video slice and browser composition
 - [moq-audio's browser backends are pinned](/quest/m0/pin-browser-audio.md) - the web studio's audio slice
 - [moq-audio's source tap is pinned](/quest/m0/pin-audio-tap.md) - the samples clap-to-measure sync times a clap from
+- [Playout writes land on positioned bus channels](/quest/m0/playout-positioned.md) - broadcast, media, and replay sound at the bus position its pictures are timed for
+- [moq-audio's positioned bus channel is pinned](/quest/m0/pin-bus-positioned.md) - `Bus::positioned`, which playout writes through
 
 ## Related
 

quest/m0/pin-bus-positioned.md

@@ -0,0 +1,20 @@
+# [XS] moq-audio's positioned bus channel is pinned
+
+## Goal
+
+The `moq` submodule includes moq-audio's `Bus::positioned`, whose
+`bus::Positioned::write(position, data)` plays a write at the bus position it
+names, from [moq-dev/moq#8273](https://github.com/moq-dev/moq/pull/8273). That
+PR also renamed `Bus::channel`/`bus::Channel` to `Bus::live`/`bus::Live`.
+
+## Plan
+
+Planned 2026-11-08 (maintainer, moq's /quest-complete), a follow-up of
+moq-dev/moq#8273, which has merged on moq's `main`.
+
+Advance it with `just upgrade-moq` in its own PR, fixing whatever the new API
+breaks here: at least the `Bus::live` rename at every `bus.channel(..)` call.
+If [moq-dev/moq#8272](https://github.com/moq-dev/moq/pull/8272) has landed by
+then, `moq_mux::timeline` is `moq_mux::sync`, and `playout.rs` follows that
+PR's migration notes too. Move no audio onto positioned writes here; that is
+[Playout writes land on positioned bus channels](/quest/m0/playout-positioned.md).

quest/m0/playout-positioned.md

@@ -0,0 +1,54 @@
+# [M] Playout writes land on positioned bus channels
+
+## Goal
+
+A broadcast, media, or replay source's sound plays at exactly the bus
+position its pictures are timed for. `rs/moq-scene`'s playout writes it
+through moq-audio's `Bus::positioned`, and its private copy of moq-audio's bus
+latency (`BUS_LATENCY`) and of the live ring's pad rule are deleted.
+
+Non-goals: the `Speaker`'s jitter targeting stays as it is, and live inputs
+(microphones, capture, guests) stay on `Bus::live`.
+
+## Plan
+
+Planned 2026-11-08 (maintainer, moq's /quest-complete), a follow-up of
+[moq-dev/moq#8273](https://github.com/moq-dev/moq/pull/8273), which added
+`Bus::positioned` for this consumer.
+
+Today `playout::Writer` writes into a live `bus::Channel` and keeps track of
+when what it wrote runs out, padding silence ahead of a frame so it sounds a
+copied 50 ms `BUS_LATENCY` later, the way moq-audio's ring would. Each source
+that plays timed audio (`source.rs`'s broadcast, media, and replay paths)
+goes through it.
+
+Decided with the maintainer:
+
+- ✅ **Studio maps an instant to a bus position** (rejected: an upstream
+  moq-audio bus clock). `Program::run_audio` (`rs/moq-studio/src/engine.rs`) already advances the bus
+  sample-exact on the program clock from `start`, so the position a frame
+  sounds at is `(sounds - start) * rate`. The writer gets the program clock's
+  `start` and the bus rate through the sources' `Context`. moq-audio's bus
+  counts frames and has no wall clock, so a clock there would guess timing
+  only the app knows.
+- ✅ **A late frame is trimmed, not dropped** (rejected: drop the whole
+  frame). `Positioned::write` fails a write any part of which is already
+  mixed, overlaps earlier audio, or lands past `bus::REACH`. The writer cuts
+  the frames whose position has passed and writes the rest at its position,
+  as today's writer does with a late part.
+
+Recommendations, not settled:
+
+- Keep consecutive writes contiguous: a frame that starts within the
+  existing one-millisecond `CONTIGUOUS` stamp rounding of where the last one
+  ended is written at that end, since a one-frame overlap from rounding fails
+  the write.
+- A frame the writer holds earlier than `REACH` ahead waits until it is in
+  reach rather than failing.
+- Test on the scene's driver-stepped bus, as `a_write_sounds_when_placed`
+  does: frames written ahead, late, and across a stall sound at the expected
+  bus positions, sample-exact.
+
+## Required
+
+- [moq-audio's positioned bus channel is pinned](/quest/m0/pin-bus-positioned.md) - `Bus::positioned`, which the writer writes through