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/gif_gs/gs_stub.sv

@@ -450,6 +450,14 @@ module gs_stub
     // out, so every existing TB / the flat production build is byte-identical
     // (zero DSP/ALM/M20K cost). Only a perspective demo build sets it 1.
     parameter bit PERSPECTIVE_CORRECT = 1'b0,
+    // GS per-vertex FOG compile gate. Default 1 = fog blend active (byte-
+    // identical to the fog build). 0 = every fog application collapses to the
+    // pre-fog color at elaboration (FOG_ENABLE is a constant), so the
+    // synthesizer constant-folds the entire fog cone — the texel*F/color*F
+    // multiplies, the FOGCOL adders/mux, AND the s2_fog_f / persp_fog_f5
+    // interpolation pipes that feed ONLY fog — off the texture->color critical
+    // path. Set 0 on board profiles whose scene fog is a functional no-op.
+    parameter bit FOG_ENABLE = 1'b1,
     // Preserve the low four XYZ2/XYZF2 screen-coordinate bits and evaluate
     // triangle coverage at the GS 12.4 pixel center.  Default off retains the
     // integer/corner raster contract used by every pre-Ch416 fixture.
@@ -5307,7 +5315,7 @@ module gs_stub
     // consumer (s2_natural_abgr pack, the PSMCT32 emit, the Z-tested emit color)
     // sees the fogged color. Gated on ras_fge → FGE=0 is byte-identical.
     logic [63:0] s2_sprite_color64;
-    assign s2_sprite_color64 = ras_fge
+    assign s2_sprite_color64 = (FOG_ENABLE && ras_fge)
                              ? {s2_sprite_color64_raw[63:32],
                                 fog_blend_abgr(s2_sprite_color64_raw[31:0], s2_fog_f, fogcol_q)}
                              : s2_sprite_color64_raw;
@@ -5326,7 +5334,7 @@ module gs_stub
     // GS per-vertex FOG — blend the post-texture/post-Gouraud RGB toward FOGCOL
     // when FGE=1 (see the SPRITE note above). Gated → FGE=0 byte-identical.
     logic [63:0] s2_tri_color64;
-    assign s2_tri_color64 = ras_fge
+    assign s2_tri_color64 = (FOG_ENABLE && ras_fge)
                           ? {s2_tri_color64_raw[63:32],
                              fog_blend_abgr(s2_tri_color64_raw[31:0], s2_fog_f, fogcol_q)}
                           : s2_tri_color64_raw;
@@ -6982,7 +6990,7 @@ module gs_stub
                 // GS per-vertex FOG — fog the held bilinear-perspective texel
                 // with its captured +5-aligned F. Gated → FGE=0 byte-identical.
                 raster_pixel_color_q    <= {32'd0,
-                    ras_fge
+                    (FOG_ENABLE && ras_fge)
                       ? fog_blend_abgr(comb_modulate ? persp_bili_mod_color
                                                      : s1_tex_color,
                                        persp_bili_fog_q, fogcol_q)
@@ -7076,7 +7084,7 @@ module gs_stub
                         // GS per-vertex FOG — fog the source RGB BEFORE the blend
                         // when FGE=1 (sprite fog is flat: s2_fog_f == ras_f0).
                         // Source alpha is NOT fogged. FGE=0 → byte-identical.
-                        s3_cs_q      <= ras_fge
+                        s3_cs_q      <= (FOG_ENABLE && ras_fge)
                                       ? {ras_color[63:32],
                                          fog_blend_abgr(ras_color[31:0], s2_fog_f, fogcol_q)}
                                       : ras_color;
@@ -7161,7 +7169,7 @@ module gs_stub
                     // F (persp_fog_f5). Gated on ras_fge → FGE=0 byte-identical
                     // to the pre-fog perspective emit.
                     raster_pixel_color_q    <= {32'd0,
-                        ras_fge
+                        (FOG_ENABLE && ras_fge)
                           ? fog_blend_abgr(comb_modulate ? modulated_s1_tex_color
                                                          : s1_tex_color,
                                            persp_fog_f5, fogcol_q)