Snapshot: fog implementation + fidelity tooling baseline (pre bilinear-clamp fix)

Per-vertex GS fog end-to-end (gs_stub emit incl. persp_emit5, gs_prim_list_feeder
XYZ2->XYZF2 on PRIM.FGE, gs_make_sh3_scheduler_fixture.py F/FGE packing), new fog
TBs, fidelity attribution tooling. Functional baseline before removing the dead
bilinear lerp8 clamps (Codex: 161-node comb loop -> -0.042ns setup fail).

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

tools/preview_scanout_ch418.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Preview and score the Ch418 captured SH3 scanout presentation map.
+
+RTL maps source_x=floor(output_x*4/5) and
+source_y=32+floor(output_y*14/15).  This host utility applies the same
+integer mapping to a 640x480 framebuffer PNG, saves the expected HDMI
+presentation, and reports RGB MAE/RMSE against a 640x480 reference.
+"""
+import math
+import sys
+from PIL import Image
+
+
+def score(a, b):
+    ae = 0
+    se = 0
+    n = a.width * a.height * 3
+    for pa, pb in zip(a.getdata(), b.getdata()):
+        for ca, cb in zip(pa, pb):
+            d = int(ca) - int(cb)
+            ae += abs(d)
+            se += d * d
+    return ae / n, math.sqrt(se / n)
+
+
+def main(argv):
+    if len(argv) != 4:
+        raise SystemExit("usage: preview_scanout_ch418.py <fb.png> <reference.png> <out.png>")
+    src = Image.open(argv[1]).convert("RGB")
+    if src.size != (640, 480):
+        raise SystemExit(f"expected 640x480 framebuffer, got {src.size}")
+    ref = Image.open(argv[2]).convert("RGB").resize((640, 480), Image.Resampling.BILINEAR)
+    mapped = Image.new("RGB", (640, 480))
+    mapped.putdata([
+        src.getpixel(((x * 4) // 5, 32 + (y * 14) // 15))
+        for y in range(480) for x in range(640)
+    ])
+    mapped.save(argv[3])
+    raw_mae, raw_rmse = score(src, ref)
+    map_mae, map_rmse = score(mapped, ref)
+    print(f"raw      MAE={raw_mae:.4f} RMSE={raw_rmse:.4f}")
+    print(f"mapped   MAE={map_mae:.4f} RMSE={map_rmse:.4f}")
+    print(f"delta    MAE={map_mae-raw_mae:+.4f} RMSE={map_rmse-raw_rmse:+.4f}")
+    print(f"wrote {argv[3]}")
+
+
+if __name__ == "__main__":
+    main(sys.argv)