fix: defer image redraw after focus

[rrbe]

Summary

• defer the focus-triggered full redraw until the terminal finishes restoring its graphics layer
• cancel a pending redraw if focus leaves again before the delay expires
• keep resize redraws immediate

Root cause

FocusGained is delivered reliably, but some terminals can still be restoring the active window or tab graphics layer at that point. The immediate clear, retransmit, and place sequence therefore succeeds at the protocol level and is then discarded by the terminal. A later keyboard event appears to fix it only because it triggers another render after activation has settled.

Verification

• make check
• make build
• iTerm2 3.6.10: original issue reproduced; fixed build retained headings across 5 focus cycles
• Ghostty 1.3.1: baseline lost the title after the first focus cycle; fixed build retained it across 5 cycles
• Kitty 0.48.2: baseline and fixed builds both retained headings across 5 focus cycles