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/Cargo.toml

@@ -10,6 +10,7 @@ rust-version.workspace = true
 [[bin]]
 name = "varde-daemon"
 path = "src/main.rs"
+required-features = ["socket-api"]
 
 [dependencies]
 varde-proto = { workspace = true }
@@ -42,8 +43,12 @@ tracing-subscriber = { workspace = true }
 [features]
 # Metered-connection detection via NetworkManager over D-Bus. On by
 # default; disable for systems without D-Bus (they run as unmetered).
-default = ["metered"]
+default = ["metered", "socket-api"]
 metered = ["dep:zbus"]
+# The unix-socket API server, systemd socket activation, and signal
+# handling. On by default; disable to embed the daemon core in-process
+# on platforms without a daemon model (iOS — ADR-0012 in redoal).
+socket-api = []
 
 [dev-dependencies]
 tempfile = "3"

varde-daemon/src/lib.rs

@@ -9,16 +9,19 @@ pub mod discovery;
 pub mod dscp;
 pub mod meta;
 pub mod metered;
+#[cfg(feature = "socket-api")]
 pub mod server;
 pub mod shaped;
 pub mod store;
 pub mod transfer;
 
+#[cfg(feature = "socket-api")]
 use anyhow::Result;
 
 /// Run a daemon with the given config until the future is dropped or the
 /// listener fails. The socket is bound before the accept loop starts, so
 /// once this future has been polled the socket path exists.
+#[cfg(feature = "socket-api")]
 pub async fn run(config: config::Config) -> Result<()> {
     let socket_path = config.socket_path.clone();
     let daemon = daemon::Daemon::open(config).await?;

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"@") {
+            // 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)?;
         }