v2.0.4

v2.0.4 — YouTube's own ⏯ drives the run

A pause or play made inside the YouTube player — its own controls, its
keyboard shortcuts, the OS media keys — now pauses and resumes the whole
run: the countdown, ON AIR and the finish notice follow it. Until now only
the transport's icon did, so a pause in the player stopped the music and
left the countdown running.

A state the app did not ask for counts once it has held 300 ms, so a
seek's passing pause, the player's echo of our own commands and a video's
end never do — the real player's sequences were measured first. Nothing
is sent back to the player, and each toggle is logged TT-YT-009.

Checked on the preview with real YouTube: a pause and a play in the frame
stop and start the countdown, a seek and a video's natural end leave it
running, and the app's own ⏯ still means what it shows.

docs/16 has the review; tests/manual/v2.0.4-live-checklist.md the live
run. Merged as #103 and released from this signed tag.