feat(qt): open the audio CD in the drive and play it

`Open > Open audio CD` reads the disc, builds an ephemeral session and lands
on its tab. Nothing touches library.db, which is the whole point of the
ephemeral path: a disc is content from outside the index that leaves with the
plastic.

It does not invent metadata. CD-DA carries no titles, and the disc measured
here reports its MCN as all zeros, so tracks are "Track N" and the album is
"Audio CD". A DiscID lookup is the obvious next step and is deliberately not
faked: a wrong title is worse than an honest number.

The feeder is the interesting part, and its liveness check took three designs
because the first two were wrong in opposite directions — both caught by
driving the app, neither by any test:

  1. `current_track_id() != play_id`: `play_streaming` posts to the audio
     thread and returns, so the id is not ours yet and the feeder retires
     before reading a sector. Nothing played at all.
  2. The same test with a wall-clock grace: a DEADLOCK. That field is stored
     only once the audio thread starts playback, which needs the initial
     buffer, which is what the feeder is filling. Waiting on it is waiting on
     ourselves; the grace expired and the track stopped mid-play.
  3. A feeder slot the audio thread plays no part in. A newer disc play claims
     it and the older thread retires on its next block. The acknowledgement
     path stays for "something else superseded us", but only AFTER the player
     confirms our id, and with no timeout — a timeout is what kills a healthy
     feeder.

A 16-bit WAV header, not `qbz_dsd`'s: that one is hardcoded to 24-bit, and
reusing it would either lie in the header or promote 16-bit samples to 24.
The shift is arithmetically lossless but it makes the DAC open at 24 bits for
a 16-bit source, which is precisely the silent conversion a bit-perfect path
must not contain. `play_streaming` also does not publish bit depth, so the CD
states 44 100/16 explicitly instead of inheriting the previous track's.

Driven end to end on the owner's disc: seven tracks with the right durations,
CD / 16-bit / 44.1 kHz badges, ALSA Direct opened at 44100 Hz in exclusive
mode, playback past 2:25 with no read errors, and two rapid skips retiring
their feeders in ~160 ms each.

Seven new strings in all eight locales.

cc

crates/qbz-qt/Cargo.toml

@@ -31,6 +31,7 @@ log = { workspace = true }
 # one of the wedges the watchdog exists to cut short. Already in the lock as
 # a transitive dep of half the tree; this only makes it direct.
 libc = "0.2"
+qbz-disc = { path = "../qbz-disc" }
 rand = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }