docs(design): add PlaybackRates design

[ronald-urbina]

Adds the PlaybackRates feature design — store state, configuration API, and
architectural decisions for consumer-configurable playback rate lists with
optional source plugin constraints.

Design summary

• playback-rates attribute on <media-player> / playbackRates prop on
  <Player.Provider> — rates live at the root
• Three store actions: setRequestedRates, setSourceRates, setPlaybackRate
• playbackRates is a derived getter (intersection of requested and source),
  never stored directly
• Source plugins constrain via setSourceRates; no overlap → empty list,
  controls disabled
• Pattern scales to quality selection and other source-constrained features

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