feat(perception): ground vision cone in human sensory physiology

- Visible half-angle 150° → 100° (200° arc), matching average human
  binocular field. Blind spot widens from 60° to 160° — genuine
  vulnerability behind the character.
- Forward half-angle stays at 60° (120° arc) — binocular overlap zone.
- Peripheral zone represents combined sensory awareness: visual
  periphery + subconscious sound/motion tracking, not just eyes.
- Per-character VisionConeConfig allows implants and perception modes
  (D-017) to widen beyond the unaugmented baseline.
- Fog shader: smooth back-edge gradient via smoothstep blend between
  Layer 2 (peripheral) and Layer 3 (deep fog), eliminating blocky
  stair-stepped tiles at the rear of the vision cone.
- D-015 updated with physiological basis and per-character config.
- D-017 updated with vision cone modification by perception modes.
- D-020 updated with protocol versioning policy: PROTOCOL_VERSION
  gates wire format, not gameplay parameters.
- DB backup after sprint 21 close.

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

client/shaders/fog.gdshader

@@ -73,33 +73,36 @@ void fragment() {
             vis = 0.0;
         }
 
-        if (vis > PERIPHERAL_LOW) {
-            // In or near vision cone
-            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 {
-                // Layer 2: Light fog (peripheral + forward edge)
-                // D-059: animated Perlin noise, 8-10s cycle
-                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);
-                // Blend from heavy fog (alpha ~0.38) to lighter fog near clear edge
-                // Taper noise to zero near the clear boundary — preserves breathing in deep
-                // peripheral but keeps the Layer 1/2 transition clean (no curly edge)
-                float noise_weight = 1.0 - smoothstep(0.5, 1.0, coverage);
-                float alpha = mix(0.38, 0.25, coverage) + noise_val * 0.05 * noise_weight;
-                COLOR = vec4(DARK_OVERLAY, alpha);
-            }
-        } else if (explored > 0.3) {
-            // Layer 3: Deep fog (previously explored, no longer in LOS)
+        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_val = texture(noise_tex, tile * 0.015 + vec2(time * 0.06, time * 0.045)).r;
+            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 alpha = mix(0.62, 0.76, noise_val);  // Fog breathes
-            COLOR = vec4(tint_color, alpha);
+            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
             COLOR = vec4(UNEXPLORED_COLOR, 1.0);