BT410: correct what the sec head is -- the colour radar, not the 3-D scene

I had it recorded as 'the 3-D scene head, this is btl4vid'.  It is not.  Port
0 carries map / headingPointer / digitalClock / numericSpeed / messageBoard /
the armour colour-mappers / the four GeneratorClusters, and its plate shows
the radar scope, the SPEED / HEADING / MISSION TIME dials and the ARMOR
DAMAGE diagram -- the pod's colour screen, the one the VDB splits out
alongside the five mono MFDs.

So sec is ordinary gauge work and is now the largest single head delta.
btl4vid is a separate deliverable this rig cannot measure at all: the 3-D
main view never enters the gauge framebuffer -- it goes down the dPL path to
the render board, and these confs run L4VIDEO=OFF, so it is not even being
produced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

restoration/source410/BT_L4/GAUGE-BLOCK.NOTES.md

@@ -854,8 +854,8 @@ big overlapping defect makes a small one measure as noise -- or worse, as
 already-optimal, which is what I recorded.
 
 WHAT IS LEFT, LARGEST FIRST:
-  * sec (325 missing / 1039 extra) -- the 3-D scene head.  This is btl4vid,
-    the world renderer, and the next big climb; the gauge work cannot move it.
+  * sec -- the COLOUR RADAR head (NOT the 3-D scene; see the 5.3.39 note).
+    Ordinary gauge work, and now the largest single head delta.
   * Comm (2099 missing / 0 extra) -- content the shipped Comm page draws that
     we never build at all.  Zero extra means nothing we draw there is wrong,
     so this is a build-it wave, not a fix-it one.
@@ -900,9 +900,20 @@ FailureTemperature / CoolantMassLeakRate) are published in HEAT.CPP.
 
 WHAT IS STILL OPEN, IN THE ORDER I WOULD TAKE THEM:
 
-  1. sec (417 missing / 670 extra) -- the 3-D scene head.  This is btl4vid,
-     the world renderer, and no amount of gauge work moves it.  The next big
-     climb, and the last one.
+  1. sec (417 missing / 670 extra) -- CORRECTED: this is the COLOUR RADAR
+     head, NOT the 3-D scene.  Port 0 carries map / headingPointer /
+     digitalClock / numericSpeed / messageBoard / the armour colour-mappers
+     / the four GeneratorClusters (widget map), and its plate shows the radar
+     scope, the SPEED / HEADING / MISSION TIME dials and the ARMOR DAMAGE
+     diagram.  It is the pod's colour screen -- the "5 mono MFDs + colour
+     radar in one 16bpp buffer" the VDB splits.  So this is ordinary gauge
+     work, and it is now the largest single head delta.
+
+     btl4vid is a SEPARATE deliverable this rig cannot measure at all: the
+     3-D main view never enters the gauge framebuffer.  It goes down the dPL
+     retained-scene path to the render board (port 0x150 wire), and these
+     confs run L4VIDEO=OFF, so it is not even being produced.  Measuring it
+     needs the render bridge, not planes.py.
 
   2. The leak-gauge hatch (~480px, and the bulk of what is left on Heat and
      the MFDs).  CORRECTED after checking both of my own guesses: