feat: seasons, episodes and audio tracks (0.5.0)

[kvachikk]

Two things a phone viewer cannot do on a film site, and now can.

Seasons and episodes

A series on a Playerjs site keeps a path — the season, the dub, the episode —
and the player draws one control per step of it. Nocturne reads those and puts
them in the top-left corner of the player, laid out across, where a thumb
reaches with the phone held sideways: the season, then the episode, each
opening a list with the one being watched marked. Picking an episode plays it
without leaving the player.

Playerjs answers api('next') and api('prev') but refuses an entry asked for
by index or by name — api('playlist') is undefined, api('play', n) throws,
api('find') returns false. So a choice is made the way its own menu makes it:
by pressing the row the player drew. The rows are read again at the moment of
the press, because choosing a season has the player rebuild every step under it
and rows held from before that are detached elements no press can reach.

A step with one entry in it is not a choice and is not drawn; a page that is a
film has no bar at all.

Audio tracks

Film sites often carry two or three dubs, and switching them from a phone was
close to impossible. The sheet grows a row for it, reading hls.js
audioTracks, dash.js getTracksFor('audio'), Shaka's audio languages,
Playerjs's own list and the audioTracks a browser exposes on the video
element. Like quality, it stays hidden when there is nothing to choose between.

Also

Quality hunted for hls.js, dash.js, Shaka and Playerjs on its own, and audio
was about to hunt for the same four. That search moves into engines.js so
both ask once, and one pass over the page's globals now answers for every
engine rather than one pass each.

Privacy

Reads and presses on elements and objects the page already published. No code
injected, nothing evaluated from a string, no network, no new permissions. The
privacy check passes on 31 files.

Checked

• 60 unit tests, eslint, prettier, web-ext lint (0 errors, 0 warnings)
• On the emulator (AOSP 16, Firefox 153), on a real series page:
  • both dropdowns draw in the top-left, the dub step correctly not drawn
  • the episode list opens with the episode being watched marked
  • picking an episode switches it in place, player never closes
  • picking a season switches the season, and its episode list replaces the old
    one; picking from it plays that episode