socket-api feature: make the daemon core embeddable in-process

Gate the unix-socket server, systemd activation, and the socket-serving
run() behind a new default socket-api feature, with required-features on
the binary. The core modules (daemon, store, transfer, discovery, meta)
stay feature-free, so platforms without a daemon model (iOS — redoal
ADR-0012) can embed Daemon directly via
cargo check -p varde-daemon --no-default-features.

Also cfg-gate the abstract-socket branch of sd_notify to Linux: abstract
socket names don't exist elsewhere, and this was the one spot keeping
varde-daemon from compiling on macOS at all.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

varde-daemon/src/server.rs

@@ -49,9 +49,19 @@ pub fn notify_ready() {
         let socket = std::os::unix::net::UnixDatagram::unbound()?;
         let bytes = path.as_encoded_bytes();
         if let Some(name) = bytes.strip_prefix(b"@") {
-            use std::os::linux::net::SocketAddrExt;
-            let addr = std::os::unix::net::SocketAddr::from_abstract_name(name)?;
-            socket.send_to_addr(b"READY=1", &addr)?;
+            // Abstract socket names are Linux-only, like systemd itself;
+            // elsewhere an @-prefixed NOTIFY_SOCKET can only be ignored.
+            #[cfg(target_os = "linux")]
+            {
+                use std::os::linux::net::SocketAddrExt;
+                let addr = std::os::unix::net::SocketAddr::from_abstract_name(name)?;
+                socket.send_to_addr(b"READY=1", &addr)?;
+            }
+            #[cfg(not(target_os = "linux"))]
+            {
+                let _ = name;
+                debug!("abstract NOTIFY_SOCKET unsupported on this platform");
+            }
         } else {
             socket.send_to(b"READY=1", &path)?;
         }