sl0ppty: a wrapped line is one line when you copy it

Select a line too long for the pane and the clipboard came back with a `\n` in
the middle of it, at whatever column the pane happened to end. Paste that into a
shell and it runs half a path.

`pane_selection_text` set `trim` and left `unwrap` at whatever a zeroed struct
gives, which is false. lib-vt's header names the exact trio the clipboard wants --
plain output with `unwrap` and `trim` both true, matching Ghostty's own
`Screen.selectionString()` -- and we had two of the three. A row boundary in the
grid is not a newline in the text: a long line occupies two rows and is still one
line, and where it breaks is a fact about how wide the pane is rather than about
what was copied.

Hard newlines are untouched, which is the half a fix for this could easily have
broken: lib-vt joins only the rows that are continuations, so three separate lines
still come back as three lines. Both directions are now asserted, because "copy
joins everything" would be a worse bug than the one being fixed and would look
identical from the inside.

`emit` is now set explicitly as well, to `GHOSTTY_FORMATTER_FORMAT_PLAIN`. It is
already zero, so this changes nothing today -- but this is the API where a zeroed
field means "whatever the enum happens to start with" (DESIGN.md's note on
`ghostty_terminal_set`), and a silently defaulted field is how the wrap flag came
to be missing. There is one call site, checked.

Verified: three checks in `tests/test_clipboard.py` -- a 50-character line printed
into a 40-column pane, dragged across the row boundary, arrives as the string that
was printed with no newline in it; three short lines keep their two newlines; and a
double-click on a word in a wrapped line brings back no newline either. 46 checks in
that file, 1795 overall, all green.

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