enum CaretBlink

Caret blink, expressed as a phase rather than a timer.

The plan called for deciding this deliberately rather than drifting into it: a blinking caret dirties a frame twice a second forever, which is the one thing that defeats idle frame-gating. The compromise here is to blink only while focused, and to hold the caret solid for a moment after any edit so it never blinks mid-typing.

The phase is a pure function of the clock, so it is shared; what is per window (WindowScope.caretLastPhase) is the last phase this window drew, since two windows both showing a caret would otherwise consume each other's flips and one of them would stop blinking.