Fog of war: unexplored is black, fades ease into it with no stepped rim (t17)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/systems/fog.md

@@ -28,10 +28,10 @@ immediately from the bot.
 
 ## Drawing and setting
 
-Both looks and the thermal lens sample fog bilinearly. Partial values get stable per-grain
-noise of about ±40, clamped to 1–255, to break up stepped edges. Unseen and fully seen
-grains are unchanged. Seen grains use the usual drawing;
-unseen grains use the out-of-world colour and smooth-look buffers, hiding their shapes.
+Both looks and the thermal lens sample fog bilinearly. Partly seen grains darken toward black
+along a smoothstep curve, with stable per-grain noise (up to about ±38 mid-fade, none at either
+end) to break up the half-resolution steps. Fully seen grains use the usual drawing; unseen
+grains are pure black (as is everything off the map) in both looks, hiding their shapes.
 Partial values dim foreground, background and trees. Bloom sources need at least 128;
 bars, flyers, rain and particles need at least 64. Hover info and the cursor label say
 only “Unexplored” below 64.