@moq/watch: expose AudioContext on the audio backend

[skirsten]

The WebCodecs decoder owns its own AudioContext but doesn't surface it past the Decoder class. Browsers create the context in suspended state when there's no user gesture, and applications need a handle on it to prompt the user (e.g. a "click to enable audio" button) and call resume() from within a real gesture handler.

Add context: Getter<AudioContext | undefined> to Audio.Backend and proxy it from the WebCodecs path in MultiBackend. The MSE backend plays through the <video> element rather than WebAudio, so its context signal stays undefined.

[coderabbitai]

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• js/watch/src/backend.ts

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Walkthrough

A context property was added to the audio backend API. The Backend interface now exposes a context getter returning AudioContext | undefined. The Mse backend implements this getter and currently returns undefined (it uses a <video> element rather than WebAudio). The AudioBackend class introduces a reactive context signal, and MultiBackend proxies that signal from the selected audio source in both WebCodecs and MSE paths.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: exposing AudioContext on the audio backend interface.
Description check	✅ Passed	The description clearly explains the motivation and implementation of exposing AudioContext, including why it's needed and how different backends handle it.
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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[coderabbitai]

Actionable comments posted: 1

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Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@js/watch/src/backend.ts`:
- Line 186: The call to effect.proxy(this.audio.context, audioSource.context)
can leave this.audio.context bound to a stale/closed AudioContext when switching
to the MSE (video) path because `#runMse` never rebinds or clears it; update
`#runMse` (or the MSE initialization path) to explicitly reset/unproxy
this.audio.context (e.g., call effect.proxy(undefined, this.audio.context) or
set this.audio.context = undefined before/after entering the MSE flow) so
consumers don’t see an old AudioContext, and ensure any existing bindings
between effect.proxy and audioSource.context are removed when switching to MSE.

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Configuration used: Organization UI

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Reviewing files that changed from the base of the PR and between 019716f and 2b67ef0.

📒 Files selected for processing (3)

• js/watch/src/audio/backend.ts
• js/watch/src/audio/mse.ts
• js/watch/src/backend.ts

[kixelated]

One thing code rabbit called out