fix(client): remove return statements from fog fragment shader

Godot 4.6 OpenGL3 compatibility mode does not support 'return' in
fragment(). The early returns on lines 68 and 82 caused silent shader
compilation failure, making the fog overlay render as a no-op — the
root cause of the Sprint 22 fog regression.

Restructured to if/else-if/else chain preserving identical logic.

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

client/shaders/fog.gdshader

@@ -63,56 +63,55 @@ void fragment() {
     vec2 tex_uv = (tile - map_offset) / map_size;
 
     // Outside known map -> unexplored
+    // Note: no early return — fragment() in OpenGL3 compat doesn't support return.
     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);
-        return;
-    }
 
     // Debug mode: render raw exploration texture (bypass fog rendering).
     // Green = EXP_VISIBLE (255), blue = EXP_EXPLORED (128), red = EXP_UNEXPLORED (0).
-    if (debug_exploration) {
-        float explored = texture(exploration_tex, tex_uv).r;
-        if (explored > 0.9) {
-            COLOR = vec4(0.0, explored, 0.0, 0.8);       // Green: currently visible
-        } else if (explored > 0.1) {
-            COLOR = vec4(0.0, 0.0, explored * 2.0, 0.8);  // Blue: explored
+    } else if (debug_exploration) {
+        float explored_dbg = texture(exploration_tex, tex_uv).r;
+        if (explored_dbg > 0.9) {
+            COLOR = vec4(0.0, explored_dbg, 0.0, 0.8);       // Green: currently visible
+        } else if (explored_dbg > 0.1) {
+            COLOR = vec4(0.0, 0.0, explored_dbg * 2.0, 0.8);  // Blue: explored
         } else {
-            COLOR = vec4(0.5, 0.0, 0.0, 0.8);             // Red: unexplored
+            COLOR = vec4(0.5, 0.0, 0.0, 0.8);                 // Red: unexplored
         }
-        return;
-    }
 
-    float vis_raw = texture(visibility_tex, tex_uv).r;
-    float vis = sample_visibility(tex_uv);
-    float explored_raw = texture(exploration_tex, tex_uv).r;
-    float explored = sample_exploration(tex_uv);
-
-    // Prevent gradient bleed into never-explored tiles (use raw, unblurred value)
-    if (explored_raw < 0.01 && vis_raw < 0.01) {
-        vis = 0.0;
-    }
-
-    if (explored < 0.01 && vis < 0.01) {
-        // Unexplored: solid near-black — information zero
-        COLOR = vec4(UNEXPLORED_COLOR, 1.0);
     } else {
-        // 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
+        float vis_raw = texture(visibility_tex, tex_uv).r;
+        float vis = sample_visibility(tex_uv);
+        float explored_raw = texture(exploration_tex, tex_uv).r;
+        float explored = sample_exploration(tex_uv);
 
-        // Zone temperature tint (D-046/D-077): subtle warm/cool/neutral per zone
-        vec3 zone_tint = texture(zone_tint_tex, tex_uv).rgb;
+        // Prevent gradient bleed into never-explored tiles (use raw, unblurred value)
+        if (explored_raw < 0.01 && vis_raw < 0.01) {
+            vis = 0.0;
+        }
 
-        // 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);
+        if (explored < 0.01 && vis < 0.01) {
+            // Unexplored: solid near-black — information zero
+            COLOR = vec4(UNEXPLORED_COLOR, 1.0);
+        } else {
+            // 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
 
-        // Soft edge between explored and unexplored (blurred to avoid staircase)
-        float exp_fade = smoothstep(0.0, 0.3, explored);
-        alpha = mix(1.0, alpha, exp_fade);
-        color = mix(UNEXPLORED_COLOR, color, exp_fade);
+            // Zone temperature tint (D-046/D-077): subtle warm/cool/neutral per zone
+            vec3 zone_tint = texture(zone_tint_tex, tex_uv).rgb;
 
-        COLOR = vec4(color, alpha);
+            // 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);
+
+            // Soft edge between explored and unexplored (blurred to avoid staircase)
+            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);
+        }
     }
 }