fix(client): move fog blur to CPU pipeline, fix GL compat bilinear on RGBA8

Replaces GPU 7×7/5×5 Gaussian blur (98 texture reads/px) with CPU-side
Gaussian blur (sigma 2.0) + 4× bilinear upscale + RGBA8 convert in
fog_state.gd. GL compatibility mode doesn't bilinear-filter R8 textures;
RGBA8 at 4× resolution resolves this. Squared exp_fade at the
explored/unexplored boundary keeps fog opaque near tile content edges,
fixing the staircase artifact. Shader now does 2 texture reads per pixel.

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

client/scripts/rendering/fog_shader.gd

@@ -32,6 +32,7 @@ func _ready() -> void:
 
 	# Create seamless noise texture for fog animation
 	var noise := FastNoiseLite.new()
+	noise.seed = 42
 	noise.noise_type = FastNoiseLite.TYPE_PERLIN
 	noise.frequency = 0.03
 	var noise_tex := NoiseTexture2D.new()