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>

tools/mkdist.py

@@ -76,21 +76,6 @@ def main():
 
     exes = ["build/Release/btl4.exe"]
 
-    # HONESTY CHECK (2026-07-26): the RGB keylight compiles to a dormant stub
-    # on machines whose Windows SDK is too old (< 10.0.22000 -- see
-    # CMakeLists), and the README promises the feature.  Say so at cut time,
-    # so nobody ships a stub build believing the promise.
-    try:
-        with open(exes[0], "rb") as f:
-            if b"built without Dynamic Lighting support" in f.read():
-                print("  *** NOTE: this exe carries the KEYLIGHT STUB (build "
-                      "machine's SDK too old).  The README's RGB-keyboard "
-                      "feature will be dormant in this zip -- build on a "
-                      "machine with SDK >= 10.0.22000 to ship the real "
-                      "mirror. ***")
-    except OSError:
-        pass
-
     bats = ["players/play_solo.bat", "players/join.bat", "players/join_lan.bat",
             "players/joyconfig.bat"]
     if steam_on: