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>

docs/architecture/fog-shader-spec.md

@@ -10,16 +10,23 @@ The fog system was simplified from the original 5-layer spec in Sprint 22 (#569)
 
 | Original D-059 Layer | Sprint 22 Status | Notes |
 |----------------------|------------------|-------|
-| 1. Clear (forward cone) | Implemented | Soft Gaussian gradient (7x7, sigma 2.0) |
-| 2. Light fog (cone gradient) | Simplified — merged into gradient | Peripheral sector removed from server (#569); Gaussian blur provides soft transition |
-| 3. Deep fog (previously explored) | Implemented — EXP_EXPLORED | Alpha 0.55-0.70 with zone temperature tint (#563) |
+| 1. Clear (forward cone) | Implemented | CPU Gaussian blur (sigma 2.0) → 4× bilinear upscale → RGBA8. Smooth sub-tile gradients. |
+| 2. Light fog (cone gradient) | Simplified — merged into gradient | Peripheral sector removed from server (#569); CPU blur + bilinear upscale provides soft transition |
+| 3. Deep fog (previously explored) | Simplified — D-015 light fog | Alpha 0.25-0.35 with zone temperature tint. Explored tiles show through light haze. |
 | 4. Unexplored + maps app | Deferred | v0.1.2+, requires mapped_tiles in ObserverSnapshot |
 | 5. Unexplored (no maps) | Implemented | Solid near-black #12141a |
 
-**Alpha values (Sprint 22, #563):**
-- Light fog zone (near cone gradient, vis 0.0-0.3): alpha 0.25-0.35, breathing ±0.05 (8-10s)
-- Deep fog zone (EXP_EXPLORED, vis≈0): alpha 0.55-0.70, breathing ±0.075 (15-20s)
-- Zone temperature tint active in deep fog zone (D-059/D-046/D-077)
+**Texture pipeline (fog_state.gd):**
+Binary 0/255 data at 1× tile resolution → CPU Gaussian blur (sigma 2.0, radius 4)
+→ `Image.resize()` 4× bilinear upscale → `Image.convert()` RGBA8.
+GL compat mode doesn't bilinear-filter R8 textures; RGBA8 at 4× resolves this.
+Exploration data is binarized (0/128/255 → 0/255) before blur to avoid a
+second gradient at the explored/visible boundary.
+
+**Alpha values (Sprint 22, D-015):**
+- Light fog zone (explored, out of cone): alpha 0.25-0.35, breathing ±0.05 (8-10s)
+- Zone temperature tint active in explored zone (D-059/D-046/D-077)
+- Explored/unexplored boundary: squared fade keeps fog opaque at tile content edge
 
 ## Overview
 
@@ -120,30 +127,27 @@ func update_from_state() -> void:
 
 File: `client/shaders/fog.gdshader`
 
-The shader determines fog state per pixel based on the visibility and exploration textures.
-Two explored sub-zones are distinguished by the blurred `vis` value (proximity to the forward cone):
+The shader reads pre-smoothed RGBA8 textures (CPU blur + 4× bilinear upscale)
+and determines fog state per pixel. No GPU-side blur — 2 texture reads per pixel.
 
 ```glsl
-// Light fog noise — fast cycle (8-10s), subtle ±0.05 breathing
-float noise_fast = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
-// Deep fog noise — slow cycle (15-20s), more pronounced ±0.075 breathing
-float noise_slow = texture(noise_tex, tile * 0.02 + vec2(time * 0.05, time * 0.035)).r;
+// Fog noise — 8-10s breathe cycle, ±0.05 symmetric around baseline
+float noise_val = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
+float fog_alpha = 0.30 + (noise_val * 2.0 - 1.0) * 0.05;  // 0.25-0.35
 
-// Light fog: alpha 0.25-0.35 (near cone gradient)
-float light_fog_alpha = 0.30 + (noise_fast * 2.0 - 1.0) * 0.05;
-// Deep fog: alpha 0.55-0.70 (far from cone, EXP_EXPLORED)
-float deep_fog_alpha = 0.625 + (noise_slow * 2.0 - 1.0) * 0.075;
-
-// Blend deep <-> light fog by proximity to cone:
-//   vis=0   (far from cone) → deep_factor=1 → deep fog color + alpha
-//   vis=0.3 (cone gradient) → deep_factor=0 → light fog color + alpha
-//   vis=0.85+ (inside cone) → clarity=1     → transparent (clear)
-float deep_factor = 1.0 - smoothstep(0.0, 0.30, vis);
-float fog_alpha = mix(light_fog_alpha, deep_fog_alpha, deep_factor);
-
-// Zone temperature tint (D-059/D-046/D-077): deep fog color = zone tint
+// Zone temperature tint (D-046/D-077): subtle warm/cool/neutral per zone
 vec3 zone_tint = texture(zone_tint_tex, tex_uv).rgb;
-vec3 fog_color = mix(DARK_OVERLAY, zone_tint, deep_factor);
+
+// Clarity ramp: transparent inside cone, light fog at edges and beyond
+float clarity = smoothstep(0.0, 0.85, vis);
+float alpha = mix(fog_alpha, 0.0, clarity);
+vec3 color = mix(zone_tint, vec3(0.0), clarity);
+
+// Explored/unexplored boundary: squared fade hides tile content edges
+float exp_fade = smoothstep(0.3, 1.0, explored);
+exp_fade *= exp_fade;  // Steeper: fog stays opaque near content edge
+alpha = mix(1.0, alpha, exp_fade);
+color = mix(UNEXPLORED_COLOR, color, exp_fade);
 ```
 
 **Zone temperature palette (D-046):**
@@ -215,11 +219,13 @@ Per tick (in _process or on snapshot signal):
 
 | Component | Budget | Estimate | Notes |
 |-----------|--------|----------|-------|
-| Visibility texture upload | 0.1ms | ~0.05ms | 400 pixels via set_pixel() |
-| Exploration texture update | 0.1ms | ~0.05ms | Incremental — only changed tiles |
-| Fragment shader (1080p) | 0.5ms | ~0.2ms | Single full-screen pass, simple math |
+| CPU Gaussian blur (1×, 40×40) | 0.2ms | ~0.1ms | Separable, sigma=2.0, radius=4 (vis: 1 pass, exp: 2 passes) |
+| Image.resize C++ (40→160) | 0.1ms | ~0.05ms | INTERPOLATE_BILINEAR, 2× textures |
+| Image.convert R8→RGBA8 | 0.1ms | ~0.02ms | GL compat bilinear requires RGBA8 |
+| Texture upload (RGBA8 160×160) | 0.2ms | ~0.1ms | 2× textures, ~200KB total |
+| Fragment shader (1080p) | 0.2ms | ~0.05ms | 2 texture reads/px (was 98 with GPU blur) |
 | Fog entity sprites | 0.2ms | ~0.05ms | 0-15 sprites, trivial draw calls |
-| **Total** | **<1ms** | **~0.35ms** | Well within D-059 budget |
+| **Total** | **<1ms** | **~0.37ms** | Well within D-059 budget |
 
 ## Files to Create