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/src/bridge/local.rs

@@ -96,17 +96,17 @@ impl SimBridge for LocalBridge {
     }
 
     fn send_handshake(&self) -> Result<(), BridgeError> {
-        use super::types::{HandshakeMessage, PROTOCOL_VERSION};
-        let msg = HandshakeMessage {
-            protocol_version: PROTOCOL_VERSION,
-        };
+        // D-192: HandshakeMessage carries no version. Send an empty marker so the
+        // client knows to begin the startup sequence (send StartupMessage next).
+        use super::types::HandshakeMessage;
+        let msg = HandshakeMessage {};
         let payload = rmp_serde::to_vec_named(&msg)?;
         let mut writer = self
             .writer
             .lock()
             .map_err(|e| BridgeError::MutexPoisoned(format!("writer: {}", e)))?;
         write_framed(writer.get_mut(), &payload)?;
-        tracing::info!("sent handshake: protocol_version={}", PROTOCOL_VERSION);
+        tracing::info!("sent handshake");
         Ok(())
     }
 

server/src/bridge/tcp.rs

@@ -158,10 +158,10 @@ impl SimBridge for TcpBridge {
     }
 
     fn send_handshake(&self) -> Result<(), BridgeError> {
-        use super::types::{HandshakeMessage, PROTOCOL_VERSION};
-        let msg = HandshakeMessage {
-            protocol_version: PROTOCOL_VERSION,
-        };
+        // D-192: HandshakeMessage carries no version. Send an empty marker so the
+        // client knows to begin the startup sequence (send StartupMessage next).
+        use super::types::HandshakeMessage;
+        let msg = HandshakeMessage {};
         let payload = rmp_serde::to_vec_named(&msg)?;
         let mut writer = self
             .writer
@@ -173,7 +173,7 @@ impl SimBridge for TcpBridge {
         let result = write_framed(stream, &payload);
         stream.set_nonblocking(true).map_err(BridgeError::Io)?;
         result?;
-        tracing::info!("sent handshake: protocol_version={}", PROTOCOL_VERSION);
+        tracing::info!("sent handshake");
         Ok(())
     }
 

server/src/bridge/text_renderer.rs

@@ -220,7 +220,6 @@ mod tests {
 
     fn make_snapshot() -> ObserverSnapshot {
         ObserverSnapshot {
-            version: PROTOCOL_VERSION,
             tick: 42,
             game_time: GameTime {
                 day: 0,
@@ -427,7 +426,6 @@ mod tests {
     #[test]
     fn empty_snapshot_no_panic() {
         let snap = ObserverSnapshot {
-            version: PROTOCOL_VERSION,
             tick: 0,
             game_time: GameTime {
                 day: 0,