sl0ppty: answer the terminal when it is asked how big it is

XTWINOPS (CSI 14/16/18 t) went unanswered. A program that draws images
needs the cell size to keep an aspect ratio; the pty's pixel fields are
the first way to get it (D17) and this query is the second, which is what
a program reaches for when those come back zeroed -- as they do on the
far side of an ssh hop. Unanswered, the query times out and the program
falls back to a guess: ida-tui guesses 10x20, and against a real 9x22
cell that is every image stretched by a fifth, which looks like a bug in
the program rather than in us.

Found alongside the png bug and from the same program, which asks
`CSI 16 t` in the same round trip as its graphics query -- so once the
png decoded, the picture would have appeared at the wrong shape.

lib-vt encodes and writes the reply once the callback says what the
numbers are; they come from the pane's own geometry, so a program is told
about the pane it is in rather than the window. Mode 2048 (the in-band
report on resize) already worked, because that one is answered from the
size passed to ghostty_terminal_resize.

Also notes in graphics.c what re-emission actually costs: we send decoded
pixels because that is the only form lib-vt keeps, so an 819KB png
reaches the client as 3.3MB of base64, once per image and again after a
reattach clears `sent`. Fine for a splash, and written down so the next
person to wonder does not have to measure it again.

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