Visual test of the B.4b handler revealed the dispatcher never fired
SelectDblLeft. OnMouseDown was only looking up Press and Hold
activations — DoubleClick bindings in KeyBindings.cs were effectively
dead code.
Adds 500ms-threshold double-click detection: tracks last-mouse-down
button + Environment.TickCount64 timestamp; a same-button press within
the threshold additionally fires ActivationType.DoubleClick for the
matching binding (Press still fires normally for the second click).
Clears the pair-state after firing so a triple-click doesn't produce
a second DoubleClick.
Tests cover same-button within threshold, beyond threshold (no fire),
different-button (no fire), and triple-click (fresh pair required).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
