#280: states is now touched by two scheduler tasks, so make it concurrent

The delayed re-check reads `states` from its own scheduled task, while
`tick` writes and prunes it. Both run on the single-threaded scheduler
Fleetd passes in today, so they are serialised — but nothing in the
class enforces that, and an unsynchronised HashMap read racing a resize
can spin a CPU forever rather than fail visibly.

`priors` and `orphanStreaks` stay plain maps: `tick` is still their only
toucher. The comment says which is which, so the next person does not
have to re-derive it.

fleetd/src/main/java/dev/ltms/fleet/health/FleetHealthMonitor.java

@@ -14,6 +14,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.function.BiConsumer;
@@ -46,7 +47,16 @@ public final class FleetHealthMonitor {
     private final long workingSuspectAfterNanos;
     private final BiConsumer<String, String> failTarget;
     private final Map<String, HealthPrior> priors = new HashMap<>();
-    private final Map<String, HealthState> states = new HashMap<>();
+    /**
+     * The live classification per member, and the only one of this class's three maps that more
+     * than one scheduler task touches. {@code tick} writes it (and prunes it to the roster);
+     * fleetd #280's delayed {@link #recheckTerminalTarget} reads it from its own separate scheduled
+     * task. Both run on the single-threaded scheduler {@code Fleetd} passes in today, so they are
+     * serialised — but nothing in this class enforces that, and an unsynchronised {@link HashMap}
+     * read racing a resize can spin a CPU forever rather than fail visibly. {@code priors} and
+     * {@code orphanStreaks} stay plain maps because {@code tick} is still their only toucher.
+     */
+    private final Map<String, HealthState> states = new ConcurrentHashMap<>();
     /**
      * CB-643: consecutive ticks on which a target looked like an orphaned delegation. The fact
      * {@link MessageService#hasOrphanedDelegation} reports is a true snapshot, but it can read true