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>

context/combat-damage.md

@@ -278,7 +278,7 @@ no-arc). The old hardwired lock, the ±30°-default cone, and the projectile pat
 fallback are all REMOVED. LMB fires lasers / RMB missiles (with SPACE/CTRL). ⚠ A view-selection
 bug was fixed en route: every
 renderable rebuild stomped `mCamera` back to the chase eye (btl4vid `mViewInside` now persists
-the chosen view — the aim camera feed and the V toggle both depend on it).
+the chosen view — the aim camera feed and the view toggle (backtick since task #68 took 'V' for rear-view) both depend on it).
 
 **WORLD STRUCTURES ARE TARGETABLE via the STATIC COLLISION TREE (task #50, 2026-07-15) [T2].**
 The `BTGroundRayHit` "terrain" tier only samples the VISUAL heightfield, which on arena1 is a