stellio: cut steady-state writes to the eMMC

Volatile journald (64M), tmpfs /tmp, no core dumps, and noatime +
commit=60 on the ext4 root so the small periodic writers (Kea leases,
timesyncd clock, speedtest textfile) coalesce. Logs are kept in RAM rather
than dropped: pppd/kea output is the main debugging tool on a gateway.

machines/stellio/configuration.nix

@@ -34,6 +34,24 @@
     clan.core.settings.name = "stellio";
     clan.core.settings.machine.description = "Flint-2 router";
 
+    # Root is ext4 on the Flint 2's eMMC: keep steady-state writes off it.
+    # Logs live in RAM (64M; the router needs them for pppd/kea debugging,
+    # unlike sirius which drops them), /tmp is tmpfs, no core dumps, and
+    # ext4 skips atime updates and flushes its journal every 60 s instead
+    # of 5 s so the small periodic writes (Kea leases, timesyncd clock,
+    # speedtest textfile) coalesce. A power cut loses at most a minute of
+    # that state, none of which matters on a gateway.
+    fileSystems."/".options = [
+      "noatime"
+      "commit=60"
+    ];
+    boot.tmp.useTmpfs = true;
+    services.journald.settings.Journal = {
+      Storage = "volatile";
+      RuntimeMaxUse = "64M";
+    };
+    systemd.coredump.enable = false;
+
     # nixpkgs' services.blocky check runs the aarch64 blocky in a
     # system.checks derivation, which an x86_64 builder without binfmt cannot
     # execute (`--max-jobs 0` -> Exec format error). Validate the same YAML