keylight: unship it -- opt-in (BT_KEYLIGHT=1), gone from all player docs

Operator decision: the player base is not on Windows 11, so the RGB keyboard
mirror should not be shipped or promised.  The feature is NOT deleted -- Cyd's
implementation stays intact behind the gate for whoever wants it -- it is
simply off by default and invisible:

  * PadRIO gate flips from opt-out (BT_KEYLIGHT=0 disables) to OPT-IN
    (BT_KEYLIGHT=1 enables).  Default boots produce zero keylight lines, and a
    stub build and a real build now behave identically for players: nothing.
  * Removed from every user-facing artifact: the README paragraph, the
    CONTROLS.md section, the CONTROLS.html footer line.
  * environ.ini template: the entry stays (it is the discoverable home for the
    opt-in) but now says OFF-unless-1, Windows 11 22H2+, and why it is
    unshipped.
  * mkdist's cut-time stub warning removed -- it existed to protect a README
    promise that no longer exists.
  * context/glass-cockpit.md records the decision, the opt-in, and the
    build-time SDK stub gate in one place.

VERIFIED: default boot logs zero keylight lines; BT_KEYLIGHT=1 engages the
feature (the stub on this machine: its one honest log line + the key map).
All three console suites pass; checkctx CLEAN.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/CONTROLS.html

@@ -814,7 +814,6 @@
     <span>BattleTech 4.11</span>
     <span>gitea.mysticmachines.com/VWE/BT411</span>
     <span>docs/CONTROLS.md · CONTROLS.txt ships with the game</span>
-    <span>RGB keyboards mirror the pod lamps · BT_KEYLIGHT=0 to disable</span>
   </footer>
 </div>
 

docs/CONTROLS.md

@@ -96,10 +96,6 @@ key MOUSE4 button 0x40        # side button = main trigger
 key MOUSE5 button 0x46        # side button = middle thumb
 ```
 
-**RGB keyboards** with Windows Dynamic Lighting mirror the panel: any key you have bound to a
-lamp button glows in the panel's own colours and flashes in step with the on-screen buttons. Turn
-it off with `BT_KEYLIGHT=0` in `environ.ini`.
-
 ---
 
 ## Cockpit layout