Add FOG_ENABLE param, prune fog cone on the board profile (fix -6.165ns setup)

The fog blend's tex_color*F multiply landed on the texture->color critical path
(u_texcache RAM -> u_tex|tex_color -> mult_126 -> raster_pixel_color_q, 46ns,
-6.165ns setup). Fog is a proven no-op for the current board scene (96.5% F=255),
so pay zero for it here.

FOG_ENABLE (default 1, byte-identical fog) added to gs_stub; each fog mux gated
(FOG_ENABLE && ras_fge) so FOG_ENABLE=0 constant-folds the entire fog cone away
(multiply, FOGCOL adders, s2_fog_f/persp_fog_f5 interp). Threaded through
top_psmct32_raster_demo_bram; set FOG_ENABLE(1'b0) on the active GS_SH3_LPDDR_FB
board arm (elsif at :1086, the one the QSF profile compiles). Fog stays default-on
everywhere else.

Verified: FOG_ENABLE=1 fog TBs unchanged PASS; FOG_ENABLE=0 (new tb_gs_fog_disabled
+ board scene TB) emits raw color = pre-fog datapath; board elaborates clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

rtl/top/de25_nano_psmct32_raster_demo_top.sv

@@ -1108,6 +1108,12 @@ module de25_nano_psmct32_raster_demo_top (
         .BIN_BUFFER_ENABLE  (1'b0),
         .HEARTBEAT_SPLICE_ENABLE (1'b0),
         .PERSPECTIVE_CORRECT(1'b1),
+        // GS per-vertex FOG pruned for the board scene: 96.5% of this frame's
+        // vertices have F=255 (fog is a functional no-op here), so gate it OUT
+        // to keep the texel*F multiply off the tex-cache->raster_pixel_color
+        // critical path (recovers the -6.165 ns setup fail). Covers the
+        // FB/_Z/_640/_SCHED/_MULTI variants, which all share this instance.
+        .FOG_ENABLE         (1'b0),
         .SUBPIXEL_XY       (1'b1),          // Ch416 — preserve native XYZ 12.4 and sample pixel centers
         .PERSP_RECIP_IDX_BITS(11),
         .BILINEAR_ENABLE    (1'b1),