refactor(simulation): drop PROTOCOL_VERSION from bridge handshake (#874, D-192)

Removes the PROTOCOL_VERSION lockstep handshake across the bridge
transports. HandshakeMessage is now an empty marker indicating the
server is ready; the client sends StartupMessage next. Field-presence
and roundtrip behavioral tests are retained — tautological
version-literal assertions were deleted in Sprint 36.

After this lands, genuine schema drift surfaces as MessagePack
missing-field errors downstream, which is the intended signal per
D-192. Client ticket #875 is blocked by this commit and will ride
the same wave; server schema changes in #878 (character_archetype
removal) also depend on PROTOCOL_VERSION already being gone.

types.rs changes are bundled with the broader #877/#878 dead-code
sweep in the following commit, since they share the same file.

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

server/tests/bridge_ipc.rs

@@ -33,7 +33,6 @@ fn snapshot_roundtrip_over_unix_socket() {
         let bridge = LocalBridge::accept(&server_path).expect("failed to accept");
 
         let snapshot = ObserverSnapshot {
-            version: PROTOCOL_VERSION,
             tick: 42,
             game_time: GameTime {
                 day: 0,

server/tests/bridge_tcp.rs

@@ -19,7 +19,6 @@ fn snapshot_roundtrip_over_tcp() {
         let bridge = TcpBridge::accept_on(listener).expect("failed to accept");
 
         let snapshot = ObserverSnapshot {
-            version: PROTOCOL_VERSION,
             tick: 42,
             game_time: GameTime {
                 day: 0,

server/tests/ipc_bench.rs

@@ -99,13 +99,9 @@ fn ipc_round_trip_latency() {
     let handshake_bytes = read_framed(&mut reader)
         .expect("read handshake")
         .expect("server closed before sending HandshakeMessage");
-    let handshake: HandshakeMessage =
+    // D-192: HandshakeMessage carries no version field. Verify it deserialises cleanly.
+    let _handshake: HandshakeMessage =
         rmp_serde::from_slice(&handshake_bytes).expect("deserialize HandshakeMessage");
-    assert_eq!(
-        handshake.protocol_version, PROTOCOL_VERSION,
-        "handshake version mismatch: server={}, client={}",
-        handshake.protocol_version, PROTOCOL_VERSION
-    );
 
     let make_input = |tick: u64| PlayerInput {
         tick,