fix(assets): correct gradient radius comment from 6-8 to 3-4 tiles

The 7x7 Gaussian kernel (sigma 2.0) produces a 3-4 tile radius
gradient, not 6-8 tiles. Header comment now matches the function
comment and actual math.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/shaders/fog.gdshader

@@ -1,7 +1,7 @@
 shader_type canvas_item;
 
 // D-059: 3-state fog shader (simplified from 5-layer by #569).
-// State 1: Clear (forward cone) — transparent, soft Gaussian gradient edge (6-8 tile spread)
+// State 1: Clear (forward cone) — transparent, soft Gaussian gradient edge (3-4 tile radius)
 // State 2: Explored (out of cone, EXP_EXPLORED) — two sub-zones by vis proximity:
 //   a) Light fog (near cone gradient): alpha 0.25-0.35, neutral dark, 8-10s breathe
 //   b) Deep fog (far from cone, vis≈0): alpha 0.55-0.70, zone temperature tint, 15-20s breathe