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/bridge_ipc.rs

@@ -77,6 +77,7 @@ fn snapshot_roundtrip_over_unix_socket() {
         debug_response: None,
             sim_errors: vec![],
             current_ticker: None,
+            settings_response: None,
         };
 
         bridge

server/tests/bridge_tcp.rs

@@ -63,6 +63,7 @@ fn snapshot_roundtrip_over_tcp() {
         debug_response: None,
             sim_errors: vec![],
             current_ticker: None,
+            settings_response: None,
         };
 
         bridge

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");

server/tests/gen_fixtures.rs

@@ -53,6 +53,7 @@ fn fixture_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot
         debug_response: None,
         sim_errors: vec![],
         current_ticker: None,
+        settings_response: None,
     }
 }
 
@@ -249,6 +250,7 @@ fn generate_msgpack_fixtures() {
         debug_response: None,
         sim_errors: vec![],
         current_ticker: None,
+        settings_response: None,
     };
     write_fixture(
         "snapshot_v2_full",
@@ -415,6 +417,7 @@ fn generate_msgpack_fixtures() {
         debug_response: None,
         sim_errors: vec![],
         current_ticker: None,
+        settings_response: None,
     };
     write_fixture(
         "snapshot_full",

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");