feat(simulation): demux inbound bridge stream — receive() + atlas routing (#969, D-225)

Replaces the fixed-type SimBridge::receive_inputs() with a tagged
receive() -> Option<Inbound>, where Inbound is Inputs(Vec<PlayerInput>) or
AtlasRequest(AtlasLayerRequest). A shared decode_inbound() demuxes a frame by
shape (msgpack array = inputs, map = atlas request) — additive, no wire change
to existing input/snapshot frames. Adds send_atlas_response() to the trait
(both TcpBridge + LocalBridge impls). receive_bridge_inputs routes inputs to
the InputQueue as before; atlas requests to a new AtlasRequestBuffer (drained
by the serve system next). Integration tests (bridge_tcp/bridge_ipc) updated to
the tagged receive(); a demux unit test covers all three branches.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/tests/bridge_ipc.rs

@@ -3,7 +3,7 @@
 use settled_reach_server::bridge::framing::{read_framed, write_framed};
 use settled_reach_server::bridge::local::LocalBridge;
 use settled_reach_server::bridge::types::*;
-use settled_reach_server::bridge::SimBridge;
+use settled_reach_server::bridge::{Inbound, SimBridge};
 use settled_reach_server::simulation::time::{DayPhase, TickRate};
 use std::os::unix::net::UnixStream;
 use std::path::PathBuf;
@@ -116,7 +116,10 @@ fn input_roundtrip_over_unix_socket() {
     let server_handle = thread::spawn(move || {
         let bridge = LocalBridge::accept(&server_path).expect("failed to accept");
 
-        let inputs = bridge.receive_inputs().expect("failed to receive inputs");
+        let inputs = match bridge.receive().expect("failed to receive") {
+            Some(Inbound::Inputs(inputs)) => inputs,
+            other => panic!("expected inputs, got {other:?}"),
+        };
 
         assert_eq!(inputs.len(), 2);
         assert_eq!(inputs[0].tick, 10);