app: cut the modals, toasts and splash out of the images under them

Overlays are painted over the panes and the compositor needs no more
than that: painted later is on top. A placement is not painted, though
-- the client's terminal draws the image over the cells, in whatever
order they arrived -- so an overlay that is only drawn is an overlay an
image sits on top of. The cheatsheet came up under a picture and was
unreadable through it, in the terminal and in the mirror both.

So a frame now records where its overlays landed, and the graphics pass
takes them out of a placement exactly as it takes out a float: the
modal's box (the cheatsheet and the picker wear the same frame), each
toast, and the splash. The list is rebuilt every compose, so a rect that
is not registered again is one that is no longer on screen.

Only the opaque things are registered. The scrim behind a modal tints
what is behind it rather than hiding it, and an image cannot be tinted
by us at all -- so a picture beside the cheatsheet stays bright while
the text around it is pushed back, which is wrong-looking but a great
deal better than the image disappearing while a modal is up.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CRz3zwZJvBvzsR9k4cj6dd

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