fix: preserve CDP mouse button state

[TheKevinWang]

Previously, releases retained the pressed buttons mask, drag movement omitted
the held button, and no event supplied CDP's force value. Consequently, page
JavaScript could see zero PointerEvent.pressure during a held drag. The two
movement-only helpers also emitted an unrelated release.

The patch:

• reports force=0.5 while a mouse button is active and force=0 on release;
• keeps buttons=1 on left-button drag movement and changes to buttons=0 on
  release;
• removes synthetic releases from Tab.mouse_move() and
  Element.mouse_move().

The 0.5 value follows the Pointer Events definition for a device that does
not support pressure: pressure is 0.5 while any button is active and 0
otherwise. Chromium's CDP exposes this directly through the optional force
field on Input.dispatchMouseEvent.

#268