Load one picture at a time on a Guition: an alert's picture before a media tile's cover (0.2.77)

A download shares ESPHome's loop with touch and drawing. An alert already
closes every card, and the card's cover with it; a media tile over the whole
page stays under the alert. Its cover now waits while the alert's picture is
announced, loading or due for a retry, and an alert picture that arrives while
a cover is on its way (or a dropped cover's download is still being closed)
starts right after it. The host harness checks all three cases on the Guition.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/CAMERA.md

@@ -29,6 +29,9 @@ what plays, with the title, the artist and the album, a progress bar and the key
 - A player without a picture (a radio station, a player that is off) keeps the player's icon in
   the cover's place; the app answers with an empty link and the screen stops asking.
 - The CYD shows the same card without the picture: its icon stands in for the cover.
+- One picture loads at a time. An alert closes an open card and its cover. Under a media tile of
+  size *Full page* the alert's picture goes first: the tile's cover waits until the alert's picture
+  is there, and a cover already on its way finishes before the alert's picture starts.
 
 ## A doorbell