fleetd #729: fold a per-boot nonce into every turnId

askSeq restarts at 0 on every daemon boot, so a turnId (session#n)
minted by one Rendezvous instance could be minted again by a later
instance and resolve to an unrelated ask. Fold a per-instance nonce
into the mint, the same way #719 fixed MessageService's ticket ids.

fleetd/src/main/java/dev/ltms/fleet/msg/Rendezvous.java

@@ -1,5 +1,6 @@
 package dev.ltms.fleet.msg;
 
+import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicLong;
@@ -101,6 +102,14 @@ public final class Rendezvous {
     /** Reverse rendezvous (CB-205): worker questions awaiting the primary's answer, keyed by {@code turnId}. */
     private final ConcurrentHashMap<String, AskWaiter> asks = new ConcurrentHashMap<>();
     private final AtomicLong askSeq = new AtomicLong();
+    /**
+     * Minted once per {@code Rendezvous} instance and folded into every {@code turnId} (see
+     * {@link #openAsk(String)}). {@link #askSeq} alone restarts at zero for every instance, so
+     * without this a {@code turnId} minted by one instance could be minted again by another and
+     * resolve to an unrelated ask with no error; this nonce makes that impossible, because an id
+     * minted by one instance can never match the id space of another.
+     */
+    private final String askBootNonce = UUID.randomUUID().toString().substring(0, 6);
     /** Per-session index of the currently-open ask, so duplicate fleet_ask calls coalesce onto one turn. */
     private final ConcurrentHashMap<String, String> openAsksBySession = new ConcurrentHashMap<>();
 
@@ -187,7 +196,7 @@ public final class Rendezvous {
         while (true) {
             AskWaiter[] minted = { null };
             String turnId = openAsksBySession.computeIfAbsent(session, _ -> {
-                String newTurnId = session + "#" + askSeq.incrementAndGet();
+                String newTurnId = session + "#" + askBootNonce + "-" + askSeq.incrementAndGet();
                 CompletableFuture<String> answer = new CompletableFuture<>();
                 AskWaiter waiter = new AskWaiter(session, answer, ownerOf(session));
                 asks.put(newTurnId, waiter);

fleetd/src/test/java/dev/ltms/fleet/msg/RendezvousTest.java

@@ -191,6 +191,53 @@ class RendezvousTest {
         assertNull(rendezvous.askOwner(t.turnId()), "a closed ask no longer reports an owner");
     }
 
+    // ── fleetd #729: per-boot nonce guards turnId against cross-instance reuse ────────────────
+
+    @Test
+    void twoInstancesMintDisjointTurnIds() {
+        Rendezvous other = new Rendezvous();
+        Rendezvous.AskTicket fromThis = rendezvous.openAsk(W);
+        Rendezvous.AskTicket fromOther = other.openAsk(W);
+        assertNotEquals(fromThis.turnId(), fromOther.turnId(),
+                "each instance mints its own id space, so even a first ask from each must differ");
+    }
+
+    @Test
+    void foreignInstanceTurnIdDoesNotResolve() {
+        Rendezvous other = new Rendezvous();
+
+        // `other` must reach the same sequence number as `rendezvous` (two asks each, the first
+        // closed so the second mints fresh), or this test passes against an empty map instead of
+        // against a colliding id.
+        Rendezvous.AskTicket firstFromThis = rendezvous.openAsk(W);
+        rendezvous.closeAsk(firstFromThis.turnId());
+        Rendezvous.AskTicket secondFromThis = rendezvous.openAsk(W);
+
+        Rendezvous.AskTicket firstFromOther = other.openAsk(W);
+        other.closeAsk(firstFromOther.turnId());
+        other.openAsk(W);
+
+        // control: the id resolves in the instance that minted it, so a false below cannot be
+        // explained by broken plumbing — only by the turnId being foreign to `other`.
+        assertTrue(rendezvous.answerAsk(secondFromThis.turnId(), "answer from this instance"),
+                "the minting instance must still resolve its own turnId");
+        assertFalse(other.answerAsk(secondFromThis.turnId(), "answer from other instance"),
+                "a turnId minted by a different instance must not resolve here");
+    }
+
+    @Test
+    void openAskStillCoalescesDuplicatesAndStillMintsDistinctIdsPerAsk() {
+        Rendezvous.AskTicket t1 = rendezvous.openAsk(W);
+        Rendezvous.AskTicket t2 = rendezvous.openAsk(W);
+        assertEquals(t1.turnId(), t2.turnId(),
+                "a second openAsk while one is open still coalesces onto the same turn");
+        assertFalse(t2.fresh(), "the coalesced ask is still reported as not fresh");
+
+        rendezvous.closeAsk(t1.turnId());
+        Rendezvous.AskTicket t3 = rendezvous.openAsk(W);
+        assertNotEquals(t1.turnId(), t3.turnId(), "two asks from the same session still get different turnIds");
+    }
+
     @Test
     void ownerPermitsIsFailClosedOnARecordAndThreeStatesAreDistinct() {
         assertFalse(Rendezvous.Owner.permits(null, null),