pod: BT_GLASS=1 is not a gate -- name the real one (BT_PLATFORM=glass)

BT_GLASS is a compile-time #ifdef; getenv("BT_GLASS") appears nowhere in the
tree.  The line rode along from the bring-up launcher into the frozen profile
and into the pod-hardware runbook, reading like the switch that turns the
glass path on.  It never did anything -- the rig worked because glass is the
DEFAULT profile when nothing is set.

Replaced with BT_PLATFORM=glass (the real spelling, so the cart does not lean
on that default) and noted why NOT BT_PLATFORM=pod: the pod profile selects
the 1995 multi-surface gauge path, which needs the NVIDIA horizontal span no
modern driver has.  Behaviourally identical -- both land gBTPlatformGlass=1.

Re-verified on the cart: 9 settings applied, GLASS profile, -fit borderless,
all three surfaces on their intended displays.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rw7No5wLTpkaUgA3ANbtZZ

context/pod-hardware.md

@@ -258,7 +258,7 @@ desktops. Two pieces were added for the cab:
 1. Probe: GPUs, every monitor's virtual-desktop rect, EDID make/model (identifies the original
    panels), serial ports (the RIO board), session type, and a PROPOSED `glass_layout.cfg` that
    assigns the six surfaces to the non-primary monitors top-to-bottom/left-to-right, centred.
-2. Drop the cfg in the game's working directory; run with `BT_GLASS=1 BT_GLASS_PANELS=1
+2. Drop the cfg in the game's working directory; run with `BT_GLASS_PANELS=1
    BT_POD_SURFACES=1 BT_GLASS_LAYOUT=load` (`=save` to persist drags instead).
 3. Read the receipts; re-assign titles to monitors in the cfg until each picture is on its panel.
 Surface→panel roles: Heat MFD = upper left (coolant), Engineering = upper centre, Comm MFD =