docs(quest): scope the v4l fork that unblocks capture-by-default

quest/m2/README.md

@@ -205,6 +205,8 @@ before format-specific metadata. Unrelated areas can proceed in parallel.
 - [Runtime QA hosts](/quest/m2/runtime-qa-hosts.md) - run exact source snapshots on accessible Linux and device hosts with retrievable debug evidence
 - [Media QA on other engines](/quest/m2/browser-media-qa-engines.md) - the media harness measures a Firefox or WebKit player over the fallback and names what each engine lacks
 - [#1310](/quest/m2/1310-why-use-the-worklet-plugin.md) - why use the worklet plugin?
+- [Capture without V4L2 bindgen](/quest/m2/capture-v4l-bindings.md) - moq-video capture builds on Linux without libclang or kernel headers
+- [Audio capture without ALSA link](/quest/m2/capture-alsa-link.md) - moq-audio capture and playback build on Linux without linking libasound
 - [Ship capture and playback](/quest/m2/cli-packaging.md) - a released moq binary can capture and play, which no distribution currently enables
 - [Windows capture parity](/quest/m2/capture-windows.md) - system audio and screen cursor capture with a settled app-capture policy
 - [Linux capture parity](/quest/m2/capture-linux.md) - Wayland window/system-audio capture with explicit display-selection and app-capture limits

quest/m2/capture-alsa-link.md

@@ -0,0 +1,37 @@
+# [S] Audio capture without runtime system libraries
+
+## Goal
+
+A released moq-audio capture/playback build carries no runtime system
+requirement on Linux: Nix provides everything needed at dev time to compile,
+and the shipped binary starts and degrades cleanly on hosts without those
+libraries.
+
+## Plan
+
+Nix already covers the dev-time side (alsa-lib, alsa-plugins, and pipewire
+are in the dev shell). What remains is the runtime linkage: cpal 0.18's
+`alsa` dependency is non-optional on Linux, so libasound becomes a load-time
+requirement and the binary refuses to start where it is absent. Follow the
+vaapi/nvidia pattern and load the system library at runtime instead, falling
+through to the next host when it is missing, so a build with the feature on
+still links and starts driverless.
+
+The current feature matrix is the contract to change: `capture` and
+`playback` pull cpal with ALSA always linked, while
+`pipewire = ["cpal/pipewire"]` and `pulseaudio = ["cpal/pulseaudio"]` activate
+cpal without either. Make the host flags require `capture` or `playback` so
+they never activate cpal alone, and document the supported combinations in
+the moq-audio feature table with the same change. If cpal cannot load ALSA at
+runtime in-tree, split that half into its own upstream quest holding the cpal
+release as a plain-text `Required` condition, and this quest requires it.
+
+Verify by building in the Nix shell, then running the shipped binary on a
+host without libasound: it starts, lists devices, and captures where a
+backend exists. Inspect the binary to prove no load-time requirement on
+libasound. The PR 3850 capture gate keeps the coverage.
+
+## Related
+
+- [Ship capture and playback](/quest/m2/cli-packaging.md) - the shippable capture milestone this work supports
+- [Optional native compilation and opt-in rendering](/quest/m0/media-features.md) - the host-flag rule this aligns with

quest/m2/capture-v4l-bindings.md

@@ -0,0 +1,28 @@
+# [S] Capture without the V4L2 bindgen build dependency
+
+## Goal
+
+moq-video `capture` builds for every builder without libclang or kernel
+headers at build time, and carries no runtime system requirement.
+
+## Plan
+
+The runtime side is already clean: the camera path drives the kernel through
+ioctls, so no system library ships in the binary. What remains is build time.
+Nix already covers in-tree builds (bindgenHook provides libclang), so vendor
+or pin the `v4l` crate with checked-in bindings and drop its bindgen build
+script, the way `moq-nvenc` does and
+[VAAPI](/quest/m2/video-vaapi.md) plans. That covers every other builder too
+(`cargo install`, Docker). Keep the `v4l` API so `capture/v4l2.rs` and the
+`v4l2` codec backend need no logic changes; fixing `v4l` fixes both features
+since both drive the same device node through it.
+
+Verify by building `just rs capture` with no libclang on PATH and by
+inspecting the binary to prove no new runtime library requirement. The PR
+3850 capture gate keeps the coverage.
+
+## Related
+
+- [Ship capture and playback](/quest/m2/cli-packaging.md) - the shippable capture milestone this work supports
+- [VAAPI encode and decode](/quest/m2/video-vaapi.md) - same pre-generated bindings pattern for libva
+- [Linux capture parity](/quest/m2/capture-linux.md) - the Linux capture milestone this work supports

quest/m2/cli-packaging.md

@@ -28,3 +28,8 @@ dependencies present, and a Docker image without them fails at build rather
 than at run. Then verify the shipped artifact runs `moq devices` and
 `moq play` on each platform, since a feature that compiles into the binary and
 then fails to open a device is the same gap one layer down.
+
+## Required
+
+- [Capture without V4L2 bindgen](/quest/m2/capture-v4l-bindings.md) - the camera path must build without libclang before every distribution can ship it
+- [Audio capture without runtime system libraries](/quest/m2/capture-alsa-link.md) - the microphone path must start without system audio libraries before every distribution can ship it