fleetd #444: separate the adapter's fallback default from the decided profile

Review found the fixture's StubLauncher fell back to 'sol' too — the
same profile place() decides — so an UNSTAMPED request could land on
spawnCount('sol') by coincidence, and the assertion's claim that the
request 'actually carried sol' was unproven. Dropping the stamping
(SpawnRequest routedReq = req) while keeping the routing survived the
test unchanged.

Fix: give the adapter 'b' as its own fallback default instead, so an
unstamped request counts against 'b', not 'sol'. Verified both
mutations against the single test in isolation:
  - drop-stamping (routedReq = req): RED, expected <sol> but was <b>
  - re-entering (return spawn(req.withProfile(decision.profile())))
    i.e. M1 from the first round: still RED, PlacementException
    naming the now-quarantined 'sol'
Restored both; full suite green at 1575 tests.

No changes to src/main — PeerLauncher's javadoc from the first round
is unchanged.

fleetd/src/test/java/dev/ltms/fleet/member/CompositePeerLauncherTest.java

@@ -1147,12 +1147,17 @@ class CompositePeerLauncherTest {
         Map<String, FleetConfig.Profile> profiles = ordered(
                 "sol", stubWorker("sol", "shared-openai"),
                 "b", stubWorker("b"));
-        StubLauncher adapter = new StubLauncher("claude", herdr, profiles, "sol", Set.of());
+        // The adapter's OWN fallback default is "b", deliberately different from the profile place()
+        // decides ("sol") — see the note below on why this must not be "sol" too.
+        StubLauncher adapter = new StubLauncher("claude", herdr, profiles, "b", Set.of());
         BackendQuarantine quarantine = new BackendQuarantine(() -> 0L, TimeUnit.MINUTES.toNanos(30));
         CompositePeerLauncher composite = new CompositePeerLauncher(List.of(adapter), "sol", profiles,
                 PlacementPolicies.fixed(), _ -> 0, null, quarantine);
 
         // 1. Resolve BEFORE anything is quarantined — sol (definition order first, fixed policy) wins.
+        // composite's own defaultProfile ("sol", the constructor arg above) never enters this: the
+        // pool poolFor(DEV) resolves to is never empty here, so place() only ever reads that field as
+        // a fallback for an empty pool, which this test does not exercise.
         PlacementDecision decision = composite.place(MemberRole.DEV);
         assertEquals("sol", decision.profile(), "sanity: nothing is quarantined yet, so sol is placed");
 
@@ -1167,9 +1172,18 @@ class CompositePeerLauncherTest {
 
         assertEquals("sol", handle.profile(),
                 "the decision from place() is honored even though sol is now quarantined");
+        // A fixture whose adapter falls back to "sol" too would let an UNSTAMPED request (one
+        // routed but never given req.withProfile("sol")) land on spawnCount("sol") == 1 by
+        // COINCIDENCE, since StubLauncher.spawn falls back to its own defaultProfile whenever
+        // req.profileName() is blank. Giving the adapter "b" as its fallback instead means only an
+        // actually-stamped request can produce this count — an unstamped one would count against
+        // "b" and this assertion would fail.
         assertEquals(1, adapter.spawnCount("sol"),
-                "the request that reached the delegate actually carried sol as its profile");
-        assertEquals(0, adapter.spawnCount("b"), "b must never be touched — the decision named sol");
+                "the request that reached the delegate actually carried sol as its profile "
+                        + "(the adapter's own fallback default is 'b', so this can't happen by accident)");
+        assertEquals(0, adapter.spawnCount("b"),
+                "b must never be touched — neither as the decision's profile nor as an unstamped "
+                        + "request's accidental fallback");
     }
 
     @Test