Confirm two playtest changes: accel+brake, and the combat computer

[cgreening]

Two behaviour changes landed during the engine work that a human should confirm
rather than a test:

1. Accel + brake together now leaves you at full speed (400), not 396.3.
   The old number was an artefact of the two inputs being applied in sequence
   at a fixed frame delta. The new one is arguably more correct and definitely
   different — it makes holding both a no-op rather than a slight brake.

2. The combat computer is no longer an overlay. It moved when screens
   gained their own contract; the information is the same but it does not sit
   over the cockpit any more.

Neither is a bug report. Both are "is this what you want?", and only flying it
answers that.