feat(simulation): error handling and recovery — panic supervision, state hash, structured errors (#85)

Protocol v17: add state_hash (desync detection) and sim_errors
(structured error reporting) to ObserverSnapshot. Add SimError,
SimErrorKind, SimErrorBuffer types. Wrap main loop app.update() in
catch_unwind — on panic, send a final SimError snapshot before exit.
Report recoverable deserialization errors to client via SimErrorBuffer.
Compute per-tick state hash from player position + NPC count + tick.
Update all test fixtures and golden files for protocol v17.

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

server/src/perception/observer/mod.rs

@@ -9,6 +9,8 @@
 use bevy_ecs::prelude::*;
 use std::collections::BTreeSet;
 
+use std::hash::{Hash, Hasher};
+
 use crate::bridge::types::*;
 use crate::knowledge::graph::filter_by_access;
 use crate::knowledge::types::{AccessRule, KnowledgeState};
@@ -103,6 +105,8 @@ pub fn compute_observer_snapshot(
     mut crisis_queue: ResMut<TriangleCrisisEventQueue>,
     sim_rng: Option<Res<SimRng>>,
     pressure_query: Query<&crate::simulation::pressure::CharacterPressure, With<PlayerCharacter>>,
+    error_buffer: Option<ResMut<SimErrorBuffer>>,
+    npc_count_query: Query<Entity, With<crate::npc::Npc>>,
 ) {
     let Ok((
         observer_entity,
@@ -396,6 +400,25 @@ pub fn compute_observer_snapshot(
         .map(TriangleCrisisEventWire::from)
         .collect();
 
+    // Compute state hash for desync detection (#85).
+    // Hash inputs: player position (x, y, z), NPC count, tick number.
+    // Uses DefaultHasher for speed — not cryptographic, just comparison.
+    let state_hash = {
+        let mut hasher = std::collections::hash_map::DefaultHasher::new();
+        time.tick.hash(&mut hasher);
+        observer_pos.x.hash(&mut hasher);
+        observer_pos.y.hash(&mut hasher);
+        observer_pos.z.hash(&mut hasher);
+        let npc_count = npc_count_query.iter().count() as u64;
+        npc_count.hash(&mut hasher);
+        Some(hasher.finish())
+    };
+
+    // Drain sim errors collected this tick (#85)
+    let sim_errors = error_buffer
+        .map(|mut buf| buf.drain())
+        .unwrap_or_default();
+
     buffer.snapshot = Some(ObserverSnapshot {
         version: crate::bridge::types::PROTOCOL_VERSION,
         tick: time.tick,
@@ -424,6 +447,8 @@ pub fn compute_observer_snapshot(
         player_knowledge,
         save_result,
         triangle_crisis_events,
+        state_hash,
+        sim_errors,
     });
 }