refactor(client): simplify fog shader to 3-layer model

Reduce from 5-layer to 3-layer fog: clear (forward cone), explored
(light overlay preserving art), and unexplored (solid near-black).
Remove peripheral sector handling from fog_state.gd.

Also preserve exploration data across texture resizes — previously,
resizing the fog texture lost all explored-tile state, causing tiles
behind the player to render as unexplored black instead of light fog.

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

client/shaders/fog.gdshader

@@ -1,11 +1,9 @@
 shader_type canvas_item;
 
-// D-059: 5-layer fog shader. Composites over world content (layers 0-4).
-// Layer 1: Clear (vision cone) — transparent, soft gradient edge
-// Layer 2: Light fog (peripheral) — desaturated + dim + animated noise, 8-10s cycle
-// Layer 3: Deep fog (explored) — near-monochrome + zone tint + breathing, 15-20s cycle
-// Layer 4: Unexplored + maps — wireframe (Sprint 6: deferred, treated as Layer 5)
-// Layer 5: Unexplored, no maps — solid near-black #12141a
+// D-059: 3-layer fog shader. Composites over world content.
+// Layer 1: Clear (forward cone) — transparent, soft gradient edge
+// Layer 2: Explored fog — light overlay indicating "not fresh", art preserved
+// Layer 3: Unexplored — solid near-black #12141a
 
 uniform sampler2D visibility_tex : filter_linear, repeat_disable;
 uniform sampler2D exploration_tex : filter_linear, repeat_disable;
@@ -18,25 +16,11 @@ uniform vec2 map_size;       // map_bounds.size (tiles)
 uniform float tile_size;     // Pixels per sim tile
 uniform float time;          // Seconds since start
 
-// D-059 fog layer colors
 const vec3 UNEXPLORED_COLOR = vec3(0.071, 0.078, 0.102);  // #12141a
 const vec3 DARK_OVERLAY = vec3(0.02, 0.02, 0.05);
 
-// D-059 thresholds
-// Forward tiles = 1.0, Peripheral = 0.706 (180/255), not-visible = 0.0
-const float CLEAR_THRESHOLD = 0.85;   // Above this: fully clear
-// Lowered from 0.55 to capture blur-extended gradient tiles in the peripheral
-// branch. The 7x7 Gaussian (sigma 2.0) produces values down to ~0.08 at
-// 3 tiles from the LOS boundary — together with bilinear interpolation this
-// yields the 6-8 sim tile soft edge specified by D-059/D-066.
-const float PERIPHERAL_LOW = 0.08;
-
-// D-059: Soft gradient via 7x7 Gaussian blur on visibility (sigma 2.0).
-// The visibility texture is 1 texel per sim tile with discrete values
-// (0 / 180 / 255). Bilinear filtering alone only smooths across ~1 tile.
-// This blur spreads the LOS boundary into a 3-4 tile radius gradient;
-// combined with smoothstep transitions, produces 6-8 tile soft edge.
-// 49 taps from a tiny (~128x128) fully-cached texture — well under 1ms.
+// Soft gradient via 7x7 Gaussian blur on visibility (sigma 2.0).
+// Spreads the cone boundary into a 3-4 tile radius gradient for soft edges.
 float sample_visibility(vec2 uv) {
     vec2 t = 1.0 / map_size;
     float sum = 0.0;
@@ -53,14 +37,11 @@ float sample_visibility(vec2 uv) {
 }
 
 void fragment() {
-    // Map UV (0-1 across ColorRect) to world pixels, then to tile coordinates
     vec2 world_px = rect_pos + UV * rect_sz;
     vec2 tile = world_px / tile_size;
-
-    // Map tile coordinate to texture UV
     vec2 tex_uv = (tile - map_offset) / map_size;
 
-    // Outside known map → unexplored
+    // Outside known map -> unexplored
     if (tex_uv.x < 0.0 || tex_uv.x > 1.0 || tex_uv.y < 0.0 || tex_uv.y > 1.0) {
         COLOR = vec4(UNEXPLORED_COLOR, 1.0);
     } else {
@@ -68,44 +49,34 @@ void fragment() {
         float vis = sample_visibility(tex_uv);
         float explored = texture(exploration_tex, tex_uv).r;
 
-        // Don't bleed gradient into never-explored tiles — keep them Layer 5
+        // Don't bleed gradient into never-explored tiles
         if (explored < 0.01 && vis_raw < 0.01) {
             vis = 0.0;
         }
 
-        if (vis > CLEAR_THRESHOLD) {
-            // Layer 1: Clear — soft edge gradient
-            // Alpha matches Layer 2 at boundary (coverage=1.0 → 0.25) for continuity
-            float edge = smoothstep(CLEAR_THRESHOLD, 1.0, vis);
-            COLOR = vec4(DARK_OVERLAY, 0.25 * (1.0 - edge));
-        } else if (vis > PERIPHERAL_LOW || explored > 0.3) {
-            // Layers 2-3: smooth blend between peripheral fog and deep fog.
-            // Same gradient approach as the forward cone — smoothstep over the
-            // blur radius eliminates stair-stepped tile edges at the back of
-            // the vision cone.
-
-            // Layer 2: peripheral fog
-            float noise_val = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
-            float coverage = smoothstep(PERIPHERAL_LOW, CLEAR_THRESHOLD, vis);
-            float noise_weight = 1.0 - smoothstep(0.5, 1.0, coverage);
-            float l2_alpha = mix(0.38, 0.25, coverage) + noise_val * 0.05 * noise_weight;
-
-            // Layer 3: deep fog (explored, no longer in LOS)
-            // D-059: near-monochrome, ~10% zone temperature tint, 15-20s breathing cycle
-            vec3 zone_tint = texture(zone_tint_tex, tex_uv).rgb;
-            float noise_deep = texture(noise_tex, tile * 0.015 + vec2(time * 0.06, time * 0.045)).r;
-            vec3 tint_color = mix(vec3(0.04), zone_tint, 0.1);
-            float l3_alpha = mix(0.62, 0.76, noise_deep);
-
-            // Back-edge gradient: blend Layer 2 → Layer 3 over the blur radius.
-            // PERIPHERAL_LOW * 3.0 ≈ 0.24 — matches the 3-4 tile Gaussian tail.
-            float back_edge = smoothstep(0.0, PERIPHERAL_LOW * 3.0, vis);
-            float alpha = mix(l3_alpha, l2_alpha, back_edge);
-            vec3 color = mix(tint_color, DARK_OVERLAY, back_edge);
-            COLOR = vec4(color, alpha);
-        } else {
-            // Layer 5: Unexplored, no maps — information zero
+        if (explored < 0.01 && vis < 0.01) {
+            // Unexplored: solid near-black
             COLOR = vec4(UNEXPLORED_COLOR, 1.0);
+        } else {
+            // Continuous blend: clear vision (vis=1) -> light fog (vis=0).
+            // The Gaussian blur creates a smooth 3-4 tile soft gradient
+            // at the cone edge — no hard boundary.
+
+            // Light fog: subtle animated overlay, preserves all art/info
+            float noise_val = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
+            float fog_alpha = 0.28 + noise_val * 0.04;
+
+            // Clarity ramp from the blurred visibility
+            float clarity = smoothstep(0.0, 0.85, vis);
+            float alpha = mix(fog_alpha, 0.0, clarity);
+            vec3 color = mix(DARK_OVERLAY, vec3(0.0), clarity);
+
+            // Soft edge between explored and unexplored
+            float exp_fade = smoothstep(0.0, 0.3, explored);
+            alpha = mix(1.0, alpha, exp_fade);
+            color = mix(UNEXPLORED_COLOR, color, exp_fade);
+
+            COLOR = vec4(color, alpha);
         }
     }
 }