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/bridge/mod.rs

@@ -91,14 +91,20 @@ impl Default for HandshakeState {
     }
 }
 
-/// Receive inputs from bridge and push to InputQueue
+/// Receive inputs from bridge and push to InputQueue.
+/// Protocol errors (malformed input) are recoverable: the frame is skipped
+/// and a SimError is pushed to the SimErrorBuffer for client reporting (#85).
 pub fn receive_bridge_inputs(
     bridge: Option<Res<BridgeResource>>,
     mut input_queue: ResMut<crate::simulation::input::InputQueue>,
     mut running: ResMut<ServerRunning>,
     handshake: Res<HandshakeState>,
+    mut error_buffer: ResMut<SimErrorBuffer>,
+    time: Option<Res<crate::simulation::time::SimulationTime>>,
 ) {
     let Some(bridge) = bridge else { return };
+    let current_tick = time.as_ref().map(|t| t.tick).unwrap_or(0);
+
     match bridge.receive_inputs() {
         Ok(inputs) => {
             if !inputs.is_empty() && *handshake == HandshakeState::Pending {
@@ -134,8 +140,22 @@ pub fn receive_bridge_inputs(
             running.0 = false;
         }
         Err(BridgeError::DeserializationWithDump(ref msg)) => {
-            // Recoverable: skip this frame's input, don't shut down
+            // Recoverable: skip this frame's input, report to client (#85)
             tracing::error!("Skipping malformed input frame: {}", msg);
+            error_buffer.push(SimError {
+                kind: SimErrorKind::ProtocolError,
+                message: format!("Malformed input frame: {}", msg),
+                tick: current_tick,
+            });
+        }
+        Err(ref e @ BridgeError::Deserialization(_)) => {
+            // Recoverable deserialization error without dump
+            tracing::error!("Skipping malformed input: {}", e);
+            error_buffer.push(SimError {
+                kind: SimErrorKind::ProtocolError,
+                message: format!("Deserialization error: {}", e),
+                tick: current_tick,
+            });
         }
         Err(e) => {
             tracing::error!("Bridge receive error: {}", e);
@@ -191,6 +211,7 @@ impl Plugin for BridgePlugin {
         app.init_resource::<SnapshotBuffer>()
             .init_resource::<ServerRunning>()
             .init_resource::<HandshakeState>()
+            .init_resource::<SimErrorBuffer>()
             .init_resource::<crate::perception::query::VisibilityGeometry>()
             .init_resource::<crate::perception::query::ActivePerceptionMode>()
             .add_systems(