GL fog: per-fragment factor -- terrain/cloud no longer escape the fog

Fog was computed per VERTEX; vertices behind the eye clamped to factor
zero, and the terrain/cloud triangles are large enough to span the
camera, so interpolation dragged the whole ground to unfogged (bright
terrain through the pre-drop blackout and the respawn reveal). The
eye-space depth now rides the varying (perspective-correct) and the
factor is computed per fragment, where clipping guarantees valid depth.

Verified offline: pre-drop hold renders solid black + HUD only (matches
the period screenshot exactly); the respawn fade shows the authentic
near-first reveal from the operator's VHS (ground clear, fog wall at
distance, sky still white).

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

dpl3-revive/patha/vrview_gl.py

@@ -84,9 +84,12 @@ void main() {
     if (u_has_col == 1) base *= in_col;                     // vertex colors modulate
     v_base = clamp(base, 0.0, 1.0);
     v_uv = in_uv;
-    v_fog = (u_fog_on == 1)
-            ? clamp((z - u_fogrange.x) / (u_fogrange.y - u_fogrange.x), 0.0, 1.0)
-            : 0.0;
+    // fog factor moves to the FRAGMENT shader: computing it per vertex
+    // zeroed it on vertices BEHIND the eye, and the terrain/cloud triangles
+    // are huge enough to span the camera -- interpolation dragged the whole
+    // ground to "unfogged" (the pre-drop blackout leak). The eye depth
+    // itself interpolates perspective-correct, so pass that instead.
+    v_fog = z;
 }
 """
 
@@ -98,6 +101,8 @@ uniform int   u_alpha_cut;
 uniform vec2  u_uvoff;        // board-side SCROLL
 uniform float u_opacity;      // DITHER n -> screen-door
 uniform vec3  u_fogcol;
+uniform int   u_fog_on;
+uniform vec2  u_fogrange;     // near, far
 in vec3 v_base;
 in vec2 v_uv;
 in float v_fog;
@@ -120,7 +125,11 @@ void main() {
     }
     // alpha only takes effect in the deferred blended pass (opaque pass
     // renders with BLEND disabled)
-    f_color = vec4(mix(rgb, u_fogcol, v_fog), clamp(v_alpha, 0.0, 1.0));
+    float fogf = (u_fog_on == 1)
+        ? clamp((v_fog - u_fogrange.x) / (u_fogrange.y - u_fogrange.x),
+                0.0, 1.0)
+        : 0.0;
+    f_color = vec4(mix(rgb, u_fogcol, fogf), clamp(v_alpha, 0.0, 1.0));
 }
 """