fix(mesh/reticulum): kill the whole daemon process group on drop

The reticulum daemon is a PyInstaller one-file binary: a bootloader parent
that forks the real Python process. `kill_on_drop`/`start_kill()` only SIGKILL
the bootloader, orphaning the forked child — which keeps holding the RNode
serial port. Across the listener's 30-min RX-stall reconnects this piled up
(observed 9 concurrent instances on a live node) all clutching /dev/ttyUSB0,
garbling the RNode so it stopped transmitting entirely.

Spawn the daemon as its own process-group leader (`process_group(0)`) and, on
drop, signal the whole group (SIGTERM for a clean RNode/socket release, then
SIGKILL as a hard backstop) so the forked child can never be orphaned.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/Cargo.toml

@@ -22,6 +22,7 @@ iroh-swarm = ["dep:iroh", "dep:iroh-blobs"]
 [dependencies]
 # Core dependencies
 tokio = { version = "1", features = ["full"] }
+libc = "0.2"   # process-group signalling for the supervised reticulum daemon
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 anyhow = "1.0"