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>

players/README.txt

@@ -129,9 +129,6 @@ of the middle of the road, or breaks each display into its own window.  The
 cockpit now scales to ANY window shape without distorting -- drag it, maximise
 it, or launch with -fit for borderless full screen.
 
-RGB KEYBOARDS: if yours supports Windows Dynamic Lighting, the keys you have
-bound to cockpit buttons glow in the panel's colours and flash along with it.
-
 This build: {VERSION}.  Private -- do not redistribute.
 {EXPIRE}Test builds STOP WORKING 14 days after they are made -- if the game
 {EXPIRE}says it has expired, just grab the operator's newest zip.