D288 M-152: static investigation, add GE_D288 near-plane probe

Read gfx_sp_tri1 against D233/D106/D271's existing guards. D233's
trivial-reject fix is ruled out as a direct cause -- GL clips well-formed
clip-space geometry correctly downstream of it. Found a new, previously
uncatalogued unguarded site: the G_CULL_BOTH backface-cull cross-product
divides raw vertex w with no epsilon guard, which can NaN/Inf near w=0
and silently skip both cull branches. Not yet confirmed against a live
repro. Added GE_D288 (cached-getenv, D250-safe) to catch the culprit
triangle's w/bbox at draw time on the next -level_20 run.

No game-logic change; port/fast3d/ diagnostic only, inert unless
GE_D288 is set.

port/fast3d/gfx_pc.cpp

@@ -1613,6 +1613,28 @@ static void gfx_sp_tri1(uint8_t vtx1_idx, uint8_t vtx2_idx, uint8_t vtx3_idx, bo
             // The whole triangle lies outside the visible area
             return;
         }
+
+        /* D288 diag (M-152): the silo intro's screen-filling stray triangle
+         * (bounded to level_20 frames ~562-741) isn't explained by this
+         * function's existing D233/D106 guards on static review -- log any
+         * triangle whose clip-space NDC bbox covers an unreasonable chunk
+         * of the screen, or whose bbox math goes non-finite, together with
+         * each vertex's w and any_behind_camera, to catch it at draw time.
+         * Remove once D288 is root-caused. */
+        static int ge_d288 = -1;
+        if (ge_d288 < 0) ge_d288 = getenv("GE_D288") != NULL;
+        if (ge_d288) {
+            float minx = std::fmin(std::fmin(v1->x / v1->w, v2->x / v2->w), v3->x / v3->w);
+            float maxx = std::fmax(std::fmax(v1->x / v1->w, v2->x / v2->w), v3->x / v3->w);
+            float miny = std::fmin(std::fmin(v1->y / v1->w, v2->y / v2->w), v3->y / v3->w);
+            float maxy = std::fmax(std::fmax(v1->y / v1->w, v2->y / v2->w), v3->y / v3->w);
+            float bbox_frac = (maxx - minx) * (maxy - miny) / 4.0f; // NDC quad is [-1,1]^2
+            if (!std::isfinite(bbox_frac) || bbox_frac > 0.15f) {
+                fprintf(stderr, "D288: tri w=(%.6f,%.6f,%.6f) bbox_frac=%.3f any_behind=%d color=(%d,%d,%d)\n",
+                        v1->w, v2->w, v3->w, bbox_frac, (int)any_behind_camera,
+                        v1->color.r, v1->color.g, v1->color.b);
+            }
+        }
     }
 
     if ((rsp.geometry_mode & G_CULL_BOTH) != 0) {