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/tests/serialization.rs

@@ -37,6 +37,8 @@ fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
         player_knowledge: None,
         save_result: None,
         triangle_crisis_events: vec![],
+        state_hash: None,
+        sim_errors: vec![],
     }
 }
 
@@ -296,6 +298,8 @@ fn snapshot_v2_fields_roundtrip() {
         player_knowledge: None,
         save_result: None,
         triangle_crisis_events: vec![],
+        state_hash: None,
+        sim_errors: vec![],
     };
 
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
@@ -350,7 +354,7 @@ fn protocol_version_constant_matches_snapshot() {
     let snapshot = test_snapshot(0, vec![]);
     assert_eq!(snapshot.version, PROTOCOL_VERSION);
     assert_eq!(
-        PROTOCOL_VERSION, 16,
+        PROTOCOL_VERSION, 17,
         "bump this assertion when protocol version changes"
     );
 }
@@ -401,6 +405,8 @@ fn all_facing_direction_variants_roundtrip() {
             player_knowledge: None,
             save_result: None,
             triangle_crisis_events: vec![],
+            state_hash: None,
+            sim_errors: vec![],
         };
         let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
         let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");