BT410: the 3-D path runs to mission load -- btl4vid's next brick is named

Run against the emulated Division card, our build reaches the mission scene
load and stops at the one method we stubbed:

  btl4vid.cpp(42): BTL4VideoRenderer::LoadMissionImplementation
                   -- btl4vid.cpp not yet reconstructed

That proves MakeVideoRenderer, the BTL4VideoRenderer ctor over the engine's
DPLRenderer, the board boot (transputer + i860 firmware, ~858K wire
transactions through the VPX HLE) and Renderer::LoadMission all work in our
build.  btl4vid is not a from-scratch climb; the engine half was always
linked and what is missing is the mission-load hook and its renderables.

Banked the reproduction rig (emulator/vidtest.conf + the host VPX env --
VPX_RESPOND alone is not enough, the iserver handshake dies without
VPX_RENDER), the authentic contract from CODE/RP/MUNGA/RENDERER.CPP:263, and
a note that the BT411 donor restructured this hook so the contract comes from
the 1995 engine and only the renderables come from the donor.

Also corrected today's earlier note: the transient 3-D frame seen during a
gauge run was NOT the dPL path.  MakeVideoRenderer returns NULL without
DPLARG and no gauge conf sets it, so no video renderer exists in those runs.

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

emulator/vidtest.conf

@@ -0,0 +1,27 @@
+[sdl]
+output=opengl
+[dosbox]
+memsize=32
+machine=svga_s3
+[cpu]
+core=normal
+cputype=pentium
+cycles=20000
+[serial]
+serial1=disabled
+serial2=disabled
+[autoexec]
+mount c "C:\VWE\TeslaRel410\emulator\image"
+c:
+set L4CONTROLS=RIO,KEYBOARD
+set L4TIMER=
+set BLASTER=A220 I5 D1 H5 P330 T6
+set VIDEOFORMAT=svga
+set BT_MECH_LOG=1
+set TEMP=c:\
+set DPLARG=/tranny~.\vrendmon.btl~/i860~.\vrnostex.mng~/device~0x150~/video~svga~/pipes~1~/qual~0x14~/system_tex~0~
+32rtm.exe -x
+BTL4REC.EXE -egg test.egg > OUTREC.TXT
+32rtm.exe -u
+echo GAME-DONE-RC=%errorlevel%
+exit