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"