When a keyDown carries Cmd/Ctrl/Opt, matches no key binding by character,
but a binding exists for the same physical key and modifiers (e.g. after
switching keyboard layout, or a Chinese/Pinyin input method changes the
character a key reports), iTerm2 now offers to turn on "Interpret key
bindings based on physical key".

Detection happens once per keyDown at the single point in
iTermApplication where no binding claimed the event. When a terminal
responder is focused the offer is an in-session announcement; otherwise
it is a modal. The offer is made at most once per launch (auto-repeat is
ignored), so it neither nags nor storms. "Don't Ask Again" suppresses it
permanently (NoSync); enabling the preference also stops it. Learn More
opens a modal with the full explanation. The Keys preferences checkbox
now stays in sync when the preference is toggled from the offer.

This supersedes the unconditional keycode-only fallback approach, which
would have silently changed matching for users who deliberately want
character-based bindings across layouts.