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/error_handling.rs

@@ -304,6 +304,7 @@ fn snapshot_with_sim_errors_roundtrips() {
             },
         ],
         current_ticker: None,
+        settings_response: None,
     };
 
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");