Glass cockpit lamps: sweep every frame so lit buttons track the sim under load

Playtesters (all on the glass surround) reported the cockpit lighting
sometimes going slow or nonexistent while the 3D view stayed perfectly
smooth.  The RIO serial/lamp stack is byte-identical to pod-proven Red
Planet (diffed L4LAMP/L4RIO/L4SERIAL), so the wiring was fine -- the defect
is the glass UPDATE CADENCE:

The lit buttons draw every frame (BTDrawCockpitPanels, reading
PadRIO::GetLampState), but the lamp STATE sweep that fills that store
(LampManager::Update -> AssertNewLampValue -> SetLamp) rides the gauge
renderer's FOREGROUND turn, which fires only once per full gauge cycle
(foreground->background->copy).  The cycle can't reach the next foreground
turn until the throttled background gauge sweep drains the ~140-instrument
active list -- and that background task starves under MP load (issue #45,
"instruments freeze while fps stays healthy").  So the lamp sweep ran
~1x/second: buttons froze / flashes stalled while the view (a separate
per-frame foreground render) stayed smooth.

Fix: BTGlassSweepLamps() runs the lamp sweep EVERY frame on the dev/glass
composite path (L4VB16.cpp, in BTDrawGaugeInset), decoupled from the gauge
cycle.  It is cheap -- deduped state pushes over ~72 lamps, no raster.  The
pod never enters BTDrawGaugeInset (real gauge hardware), so its authentic
bandwidth-paced serial lamp cadence is untouched.  BT_GLASS_LAMP_SWEEP=0
restores the authentic once-per-cycle behaviour.

Verified: Release links clean (only the 40 tolerated /FORCE externals);
glass surround boots and runs, lamp sweep pushes state (43 [lamp] pushes,
buttons lit), ~60 fps, no crash.  The under-load win is structural (the
sweep is now an unconditional per-frame call); true MP-starvation repro
needs multiple nodes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

context/decomp-reference.md

@@ -427,6 +427,7 @@ default-ON (`'0'` disables).
 | `BT_SECTOR_LOG` | SectorDisplay Make/Execute |
 | `BT_NET_TRACE` | `[net-tx]/[net-rx]/[net-upd]` MP tracing |
 | `BT_DEV_GAUGES` | render the 6 pod MFD surfaces in a separate dev window |
+| `BT_GLASS_LAMP_SWEEP` | `0` = restore authentic once-per-gauge-cycle cockpit-lamp updates; default (on) sweeps the lamps EVERY frame in the dev/glass composite path so the lit buttons track the sim even when the throttled background gauge sweep starves under MP load (`L4VB16.cpp` `BTGlassSweepLamps`; the "lighting slow/nonexistent" fix — glass-only, pod serial cadence untouched) |
 | `BT_AIM="x y"` | pin the reticle crosshair (reticle coords) — headless aim harness |
 | `BT_AIM_LOG` | `[pick]` ray/box/hit diagnostics (Mech::PickRayHit) |
 | `BT_FIRE_ARC=<deg>` | OPT-IN external-camera fire-arc clamp (unset = authentic no-arc) |

context/gauges-hud.md

@@ -111,6 +111,21 @@ viewport), and `gWindowAspect` = the view rect's on-screen aspect (`BTWorldAspec
   the work area). Green tint tunable via `BT_COCKPIT_TINT=RRGGBB` (default `0x27E8`). Labels are a
   lazy GDI-baked MANAGED atlas (survives device reset). Renders in ALL builds; only the PadRIO
   click/lamp seam is BT_GLASS-gated. Full detail: `docs/GAUGE_COMPOSITE.md`.
+- ⚠ **Cockpit-lamp LATENCY under MP load (2026-08-04, glass playtest fix) [T2 runtime-verified].**
+  The lit cockpit buttons are drawn every frame (`BTDrawCockpitPanels`, reading `PadRIO::GetLampState`),
+  but the lamp STATE sweep that fills that store — `LampManager::Update` → `AssertNewLampValue` →
+  `SetLamp` — authentically rides `GaugeRenderer::ExecuteForeground`, which fires only **once per full
+  gauge cycle** (foreground→background→copy). The cycle can't advance to the next foreground turn
+  until the THROTTLED background gauge sweep drains the ~140-instrument active list, and that
+  background task starves under load (issue #45 — "instruments freeze while fps stays healthy"). So on
+  a busy MP mission the lamp sweep ran ~1×/s: **buttons froze / flashes stalled while the 3D view (a
+  separate per-frame foreground render) stayed smooth** — the "lighting slow or nonexistent" reports
+  (all testers on glass surround). Fix: `BTGlassSweepLamps()` runs the lamp sweep EVERY frame on the
+  dev/glass composite path (`L4VB16.cpp`, in `BTDrawGaugeInset`) — cheap (deduped pushes, ~72 lamps),
+  decoupled from the gauge cycle. The pod never enters this path (real gauge hardware), so its
+  bandwidth-paced serial lamp cadence is untouched. Kill switch `BT_GLASS_LAMP_SWEEP=0`. NB the RIO
+  serial/lamp stack itself is byte-identical to pod-proven Red Planet (`L4LAMP.cpp`/`L4SERIAL.cpp`
+  diffed) — the defect was purely the glass update CADENCE, not the lamp wiring.
 
 ## MP gauge-window FREEZE = dangling bindings + permanent SEH disable (Gitea #12, 2026-07-19) [T2 log-convicted]
 The live-MP "dev-gauges window froze entirely mid-session" incident (issue #12) is NOT a