Searchlight beam: the btfx brighten material class + BT_SPOT_SELF rig

SPOT.BGF decoded: a 7-vert cone from the mount, ~50u forward and ~35deg DOWN
(a ground-pool lamp, not an air beam), verts tinted cyan-white, material
class 'brighten' smuggling its additive factor in DIFFUSE.r (0.25) with a
warm emissive on the night page.  The loader had never met the class -- it
drew as an opaque dark-red blob.  Now: brightenFactor parsed (name-gated to
brighten*), batch -> L4DRAWOP.brightenAlpha, drawn in the blend pass as an
additive veil (dest += vertexRGB x factor), unlit.  [T3] tint compose
(vertex cyan vs night emissive warm) noted in the draw branch -- field
eyeball accepted the current look.

BT_SPOT_SELF=1 (bench-only): builds the cone on the own-cockpit tree so the
BT_CAM=face view can inspect it solo.  KB: view toggle is BACKTICK ('V' is
the rear-view hold since #68 -- the toggle skips bound keys); stale V-toggle
claims swept.

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

game/reconstructed/btl4vid.cpp

@@ -210,7 +210,12 @@ void
 						// is how a night mission starts on nosearchlightfog.
 						tree.searchFogCache[tree.searchLightCount] =
 							(*attrs[sl] == 0);
-						if (!tree.searchIsCockpit)
+						// BT_SPOT_SELF=1 (bench-only): build the cone on the
+						// OWN-cockpit tree too, so the V-chase camera can
+						// eyeball it solo.  Authentic builds never cone the
+						// cockpit tree (the 1995 view-1 branch stashes attrs
+						// only) -- keep this env off outside look benches.
+						if (!tree.searchIsCockpit || getenv("BT_SPOT_SELF"))
 						{
 							// NB: the renderer-level LoadObject wrapper (@00498448) is
 							// still a btstubs no-op; load through the d3d route the