Test daemon mode: StopDaemon lifecycle and crash recovery

- StopDaemon: a search caches a live daemon; StopDaemon clears it (and any
  in-flight pipe), is idempotent, and a later search respawns cleanly.
- Recovery: a GAP refiner callback that errors mid-search leaves the vole
  process parked (inflight set); the next search warns, reaps it, forks
  afresh, and still answers correctly. This mirrors a user hitting an error
  inside vole and quitting the break loop.

Author: ChrisJefferson

tst/daemon.tst

@@ -0,0 +1,73 @@
+#@local boom
+gap> START_TEST("daemon.tst");
+
+#
+# Daemon mode keeps one vole process alive between searches and reuses it.
+# _Vole.daemon caches the live pipe; _Vole.inflight holds the pipe of a search
+# in progress (cleared on clean completion, left set if a call escapes).
+#
+gap> LoadPackage("vole", false);;
+gap> _Vole.UseDaemon;
+true
+
+#
+# StopDaemon lifecycle.
+#
+gap> _Vole.StopDaemon();;
+gap> _Vole.daemon;
+fail
+
+# A search leaves a live daemon cached for reuse; nothing left in flight.
+gap> Size(VoleFind.Group([VoleRefiner.SetStab([1, 2, 3]), SymmetricGroup(6)]));
+36
+gap> _Vole.daemon <> fail;
+true
+gap> _Vole.inflight;
+fail
+
+# StopDaemon tears the daemon down; both slots clear.
+gap> _Vole.StopDaemon();;
+gap> [_Vole.daemon, _Vole.inflight];
+[ fail, fail ]
+
+# StopDaemon is idempotent: stopping again when nothing runs is a no-op.
+gap> _Vole.StopDaemon();;
+gap> _Vole.daemon;
+fail
+
+# A search after a stop spawns a fresh daemon and still answers correctly.
+gap> Size(VoleFind.Group([VoleRefiner.SetStab([1, 2, 3]), SymmetricGroup(6)]));
+36
+gap> _Vole.daemon <> fail;
+true
+
+#
+# Recovery: a GAP refiner callback that crashes mid-search leaves the vole
+# process parked (inflight set). The next call must notice, warn, reap it,
+# fork a fresh vole, and still answer correctly -- rather than wedging the
+# session. This simulates a user hitting an error inside vole and quitting
+# the break loop.
+#
+gap> SetInfoLevel(InfoVole, 1);;
+gap> boom := Objectify(BTKitRefinerType, rec(
+>          name := "Boom",
+>          largest_required_point := 3,
+>          constraint := Constraint.Stabilise([1, 2, 3], OnSets),
+>          refine := rec(
+>              initialise := function(ps, l) Error("boom from refiner callback"); end)));;
+gap> VoleFind.Group([boom, SymmetricGroup(6)]);
+Error, boom from refiner callback
+gap> _Vole.inflight <> fail;
+true
+
+# The next search reaps the crashed process (with a warning) and recovers.
+gap> Size(VoleFind.Group([VoleRefiner.SetStab([1, 2, 3]), SymmetricGroup(6)]));
+#I  Previous vole call didn't exit cleanly, restarting vole
+36
+gap> _Vole.inflight;
+fail
+gap> SetInfoLevel(InfoVole, 0);;
+gap> _Vole.StopDaemon();;
+
+#
+gap> STOP_TEST("daemon.tst");