terminals: ask Ghostty only once focus has settled

across a Space switch (a full screen window) tmux sees the new client
focused about 0.3 s before Ghostty's front window changes, so learn paired
the client with the terminal that had focus. live, raising a full screen
window and back crossed the two pairings, and from then on every jump
raised the wrong window (old binary: 0 of 10 jumps right).

learn now waits a second before asking Ghostty, then still requires the
client to be focused. it also drops pairings of clients no longer attached,
which piled up with every closed window.

e2e step 20: Ghostty lagging behind tmux's focus, crossed pairings healing,
pruning; the old learn fails all five. live, two Ghostty windows, one full
screen: 16 of 16 jumps right, starting from crossed pairings.

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