feat(simulation): SQLite settings storage with IPC protocol v20 (#627)

Per-player settings via rusqlite (bundled, zero runtime dep). Server
owns the settings DB; client sends ChangeSettings commands over IPC.
Extensible key-value with typed columns (String/Int/Float/Bool).
Protocol bumped to v20 with settings_response in ObserverSnapshot.

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

server/tests/serialization.rs

@@ -41,6 +41,7 @@ fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
         debug_response: None,
         sim_errors: vec![],
         current_ticker: None,
+        settings_response: None,
     }
 }
 
@@ -304,6 +305,7 @@ fn snapshot_v2_fields_roundtrip() {
         debug_response: None,
         sim_errors: vec![],
         current_ticker: None,
+        settings_response: None,
     };
 
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
@@ -358,7 +360,7 @@ fn protocol_version_constant_matches_snapshot() {
     let snapshot = test_snapshot(0, vec![]);
     assert_eq!(snapshot.version, PROTOCOL_VERSION);
     assert_eq!(
-        PROTOCOL_VERSION, 19,
+        PROTOCOL_VERSION, 20,
         "bump this assertion when protocol version changes"
     );
 }
@@ -413,6 +415,7 @@ fn all_facing_direction_variants_roundtrip() {
         debug_response: None,
             sim_errors: vec![],
             current_ticker: None,
+            settings_response: None,
         };
         let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
         let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");