- Add missing return after announcing Black for low-brightness colors,
  which was causing the real (dim) color to be announced again right
  after, double-firing the color-changed event.
- Fix operator precedence in avgBias calculation: parenthesize the
  sum before dividing by 3, so red/green bias are averaged along with
  blue instead of being added in full.
- Route the low-brightness Black target through the same weighted-
  averaging step as any other color instead of returning early before
  it. The early return fixed the double-fire but also skipped
  smoothing for every transition into darkness, causing an abrupt
  hard cut to black instead of a smooth fade whenever the sampled
  brightness dropped below the cutoff.

The cdbc29b fix made the black-cutoff branch fall through to weighted
averaging so in-capture transitions stay smooth, but that same branch
is used by every power-off route (quit, power toggle, sleep/lock,
session logoff). With weighting enabled, the off command's black got
blended with the last on-screen color instead of sent as-is, and
since it's the final color before disconnect the device stayed lit.
Set manualMode before sending PowerOffCommand, same as other
immediate-color overrides, to bypass weighting for these.