docs: volume & muted persistence design

[ronald-urbina]

Design doc for #1428. Captures the agreed approach: pass a StorageAdapter directly to createVolumeFeature(adapter?) rather than threading it through PlayerTarget. No HTML attributes, no storageKey, no ProviderMixin changes just a plain adapter constant wired in at player element definition time.

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Note

Low Risk
Documentation-only change with no runtime or API impact until the described implementation lands.

Overview
Adds a draft design doc for issue #1428 describing how volume and muted preferences will persist across reloads.

The proposed approach introduces a StorageAdapter interface in core (no window), refactors volumeFeature into createVolumeFeature(adapter?) with restore-before-sync and persist on volumechange, and wires a try/catch localStorageAdapter into built-in HTML/React player presets by default (opt-out via custom feature arrays). It explicitly rejects threading storage through PlayerTarget, HTML attributes, or storageKey, documents vjs-pref-volume / vjs-pref-muted keys, and covers edge cases (private browsing, iOS volume, shared keys across players, stored prefs vs <video muted>, SSR).

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