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/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?;