make lan: a fresh distd always serving the .lan store on the LAN

Rebuilds, frees the port (stale servers from earlier sessions squat on
it), and restarts detached on 0.0.0.0:8787; the .lan/ store and its log
are long-lived and gitignored.

.gitignore

@@ -9,5 +9,8 @@ build/
 # scratch store roots / fixtures created by tests (never /tmp)
 .sx-tmp/
 
+# the long-running LAN demo store served by `make lan`
+.lan/
+
 # sx compiler caches (object cache for #import c units etc.)
 .sx-cache/