Key-command audit: the up arrow was the abort key; bombs disarmed

Full input-surface audit of the PC keyboard channel. The arcade abort
was the typed ampersand character (0x26), but the Win32 port feeds
WM_KEYUP VIRTUAL-KEY codes into the same channel - and VK_UP is also
0x26. The hat look-up key aborted the mission: that was every mystery
abort across the multiplayer test rounds. Likewise the E key (0x45,
the right pedal!) dumped the event queue on every release.

Disarmed: plain 0x26 and E are swallowed at the L4 layer; the abort
answers only to the deliberate Alt+Q chord (translated to the legacy
engine code, so APP.cpp is untouched); the debug toggles (wireframe
Alt+W, predator vision Alt+V, frame dump Alt+F, perf stats, event
queue on Alt+E now) arm only with RP412DEVKEYS=1. The cheat-string
manager has no PC-keyboard strings and the trace-log keys are
compiled out of release - both inert.

Verified live: a volley of VK_UP releases mid-race leaves the mission
running; Alt+Q aborts on demand. Docs and the dist README updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/STEAM-3-MACHINE-TEST.md

@@ -61,10 +61,13 @@ this server-side; test machines need the client setting.)
 
 ## The abort key
 
-The ampersand key (Shift+7) is the arcade mission-abort. It ends YOUR
-mission on the spot - score banked, back to the lobby room. If the
-host presses it, the race ends for everyone (members return to the
-room too). Every "crash on keypress" so far has been this key.
+**Alt+Q** aborts your mission deliberately - score banked, back to the
+lobby room. If the host presses it, the race ends for everyone.
+
+(History: the legacy abort was the '&' character, and the Win32 key
+channel made the UP ARROW - the hat look key! - collide with it. That
+was every mystery abort in rounds five and six. Plain arrow keys are
+now safe; debug keys like wireframe/frame-dump need RP412DEVKEYS=1.)
 
 ## Notes and knowns