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>

decisions/architecture.md

@@ -69,6 +69,7 @@ Technical foundation decisions that constrain implementation: engine, client-ser
   - `ObserverSnapshot`: the only data structure crossing the boundary. Contains visible entities, fog state, sound events, monologue triggers, HUD widget data. Variable shape per character build.
   - `PlayerInput`: semantic actions (MoveNorth, Interact, UsePerceptionMode), not raw key events. Timestamped for deterministic processing.
   - `SimBridge` trait: abstracts transport. `LocalBridge` (subprocess, channels) and `NetworkBridge` (TCP, MessagePack) implement the same interface.
+- **Protocol versioning policy:** `PROTOCOL_VERSION` gates **wire format compatibility** — field names, types, message structure. Bump when deserialization would fail (added/removed/renamed fields, new enum variants, changed types). Do NOT bump for gameplay parameter changes that affect what data flows through the same format (vision cone angles, NPC behavior, map layout, balance tuning). Per-character variation (D-015 cone config, D-017 perception modes) means these parameters differ between entities on the same server simultaneously — they are game state, not protocol. The client renders whatever `ObserverSnapshot` the server sends; it has no awareness of cone angles or perception mode configuration.
 - **Kill switch:** If no working prototype (character + fog + one NPC) exists by week 8 of development, pivot to pure Godot. If bridge/sync code exceeds game logic for 3 consecutive sprints, the architecture tax is too high.
 - **Development sequence:**
   1. Build Rust simulation as standalone binary (testable via terminal/logs)