0.6.0

One release for every Art Computer in the field.

• Both firmware generations. The pairing request is the one 0.5 sends. An
  FF1 on FF OS with display grants answers with a grant (relay v2, as in
  0.5); an FF1 on older FF OS answers with a relayer session, which 0.5
  discarded with device_unsupported and 0.6 keeps and casts with, through
  the legacy relayer's HTTPS POST /api/cast, as 0.4 did.
• EphemeralBrowserSession is a union discriminated by kind:
  "displayGrant" ({ deviceId, ctl, expiresAt?, persistent }) or
  "relayerSession" ({ sessionId, expiresAt?, persistent, relayerBaseUrl? }).
  The session's token stays in localStorage and is never returned.
• Storage precedence. A stored grant first (IndexedDB), then a stored
  session (localStorage, under ephemeralBrowserSessionStorageKey(origin),
  the 0.4 key and record), else pair. A new grant replaces a stored session.
  Sessions 0.4 stored are used, no longer removed.
• relayerBaseUrl is an option again on DisplayDp1PlaylistOptions and
  mountDisplayOnArtComputerButton (a value or a function), with no default;
  the session's own relayerBaseUrl wins. DisplayDp1PlaylistOptions takes
  fetchImpl again.
• Relayer answers. 400 {"error":{"id":1003}} (no FF1 on the session's
  channel: the FF1 updated, or it is off) is device_unrecognized: the
  session is kept and the button offers "Pair again?". 401/403 is
  session_rejected, and the session is deleted.
• unpairArtComputer() and clearStoredEphemeralBrowserSession(origin) also
  forget a stored session.
• Removed the 0.5 deprecated aliases: mountPlayOnArtComputerButton,
  PlayOnArtComputerButtonOptions, PlayOnArtComputerButtonHandle,
  PlayError, PlayErrorCode, the ff-ac-play, ff-ac-play-button and
  ff-ac-play-status class names, and the play.js release asset. Use the
  Display names and ff-ac-display* classes.
• Removed legacySessionStorageKey: use ephemeralBrowserSessionStorageKey
  (the same key).
• On a session's 1003 the button says "Could not reach your Art Computer. If
  it is on and was updated recently, pair again." and its "Pair again?" pairs
  without deleting the session first: if that pairing fails, the session
  still works once the FF1 is back. A browser whose IndexedDB is missing or
  will not open still casts with a stored session; only a new pairing needs
  IndexedDB.
• A relayer-session cast honours timeoutMs (one deadline over the request,
  its retry and the body read), like a grant cast.

feral-file/feral-file#3551, #33.