fix(asr): [PKG-ASR-07] apply native playback volume only once

packages/artplayer-plugin-asr/ARCHITECTURE.md

@@ -34,6 +34,13 @@ continues through its playback gain to the destination. A source switch reuses t
 direct source and replaces the recorder; a captured-stream fallback is closed and
 reacquired because captured tracks can end on source changes.
 
+The playback gain stays at unity. Native media-element source output already
+reflects the video's volume and mute; applying the same volume again would turn
+50% volume into 25% output amplitude. PCM also follows the main video's volume
+and becomes silent when that element is muted. Native analyser tests cover the
+linear output ratio, mute, stop and resume on Chromium/Firefox; they do not measure
+physical speakers or establish Safari/device support.
+
 `stop()` returns `Promise<void>` and releases ASR work. An already displayed
 subtitle retains its original auto-hide deadline; obsolete callbacks cannot add timers.
 It retains playback subscriptions and allows explicit append or a later play.
@@ -85,6 +92,9 @@ From the repository root:
 - `yarn test:browser test/browser/asr-audio.spec.js`: local AAC, actual Worklet,
   published/current core, pause/resume, source switch, stop/restart and destroy.
   Set `ARTPLAYER_ASR_ARTIFACT` to an actual main/legacy file to test that artifact.
+- `yarn test:browser test/browser/asr-playback.spec.js`: frozen 2.1 output
+  attenuation versus candidate linear volume, mute and stop/resume. A native
+  unity analyser measures the gain-to-destination route with local audio.
 - `yarn typecheck`: includes the package strict/noUncheckedIndexedAccess project.
 - `yarn test:asr-types-package`: packs and installs core/ASR outside the workspace,
   checks frozen 2.0/2.1 consumers and candidate declarations in strict compiler