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>

server/src/perception/vision_cone.rs

@@ -1,9 +1,21 @@
 //! Vision cone system (D-015)
 //!
-//! Modulates raw shadowcast output with direction-dependent sectors:
-//! - Forward: full LOS range, full detail (~120 degree arc)
-//! - Peripheral: reduced range, dimmer (~90 degrees each side)
-//! - Behind: blind (excluded from output)
+//! Modulates raw shadowcast output with direction-dependent sectors
+//! grounded in human sensory physiology:
+//!
+//! - Forward: 120° arc (60° half-angle) — binocular overlap zone where
+//!   both eyes converge. Stereoscopic depth, high acuity. Full LOS range.
+//! - Peripheral: extends to 200° arc (100° half-angle) — combined sensory
+//!   awareness envelope. Beyond pure visual acuity (~120°), this includes
+//!   subconscious tracking of sound cues, movement in the corner of the
+//!   eye, and ambient sensory input. Reduced range, dimmer rendering.
+//!   The fog shader gradient (D-059) is aligned to this outer edge.
+//! - Behind: 160° blind arc — no sensory awareness without explicit cues.
+//!   You can be snuck up on. Monologue system (D-016) bridges this gap
+//!   when the character's other senses fire.
+//!
+//! Reference: binocular field ~200° horizontal, overlap ~120°,
+//! far peripheral limit ~100-105° per eye (Wikipedia, NCBI NBK220).
 //!
 //! Y-down convention: North = (0, -1)
 
@@ -22,16 +34,23 @@ impl Default for Facing {
     }
 }
 
-/// Vision cone configuration per D-015
+/// Vision cone configuration per D-015.
+///
+/// Angles grounded in human visual field physiology:
+/// - forward_half_angle (60°): binocular overlap — where both eyes converge,
+///   providing stereoscopic depth and high acuity.
+/// - visible_half_angle (100°): average human binocular field limit — the
+///   outermost boundary of peripheral vision. The fog shader's transparency
+///   gradient is aligned to this edge.
 pub struct VisionConeConfig {
     /// Maximum vision range for forward sector (in tiles)
     pub forward_range: i32,
     /// Maximum vision range for peripheral sector (shorter than forward)
     pub peripheral_range: i32,
-    /// Half-angle of forward cone in radians (~60 degrees = 120 degree arc)
+    /// Half-angle of forward cone in radians (60° = 120° arc, binocular overlap)
     pub forward_half_angle: f32,
-    /// Half-angle of total visible cone in radians (~150 degrees = 300 degree arc)
-    /// Tiles beyond this are in the blind spot
+    /// Half-angle of total visible cone in radians (100° = 200° arc, peripheral limit)
+    /// Tiles beyond this are in the blind spot (~160° behind)
     pub visible_half_angle: f32,
 }
 
@@ -40,8 +59,8 @@ impl Default for VisionConeConfig {
         Self {
             forward_range: 20,
             peripheral_range: 12,
-            forward_half_angle: std::f32::consts::FRAC_PI_3, // 60 degrees = 120 degree arc
-            visible_half_angle: 5.0 * std::f32::consts::FRAC_PI_6, // 150 degrees = 300 degree arc
+            forward_half_angle: std::f32::consts::FRAC_PI_3, // 60° = 120° arc (binocular overlap)
+            visible_half_angle: 100.0_f32.to_radians(),      // 100° = 200° arc (peripheral limit)
         }
     }
 }
@@ -258,7 +277,7 @@ mod tests {
         assert!(!cone.is_empty());
 
         // Tile directly south (same x, far behind) should be in blind spot
-        // The blind spot is the 60 degrees directly behind
+        // The blind spot is ~160 degrees behind (beyond 100° peripheral limit)
         let has_direct_south_far = cone.iter().any(|&(x, y, _)| x == 5 && y >= 10);
         assert!(
             !has_direct_south_far,