fleetd #453: document the override obligation on PeerLauncher.defaultProfileFor/place

Decision: leave both as default methods (option 1), not abstract. Neither
default is a live defect today — HerdrPeerLauncher is the sole single-profile
implementer and the degenerate answer (ignore role, always defaultProfile())
is correct for it. Making them abstract would force ~10 boilerplate one-line
overrides across 5 unrelated PeerLauncher test doubles (NeverSpawnsLauncher,
RaceLauncher, NoResumeLauncher, ClearContextSpyLauncher, LazyIdLauncher) that
never call either method, for a risk that is speculative (no multi-profile
HerdrPeerLauncher subclass exists or is planned).

Strengthens both javadocs with an explicit MUST-override warning and cross-
references HerdrPeerLauncher.spawn(SpawnRequest, PlacementDecision)'s existing
#450 javadoc, which already names both methods as "unoverridden here" and
ties that to being a single-profile adapter -- the concrete place a future
multi-profile launcher author would read, since #450 made that method
abstract and any subclass must write its body.

fleetd/src/main/java/dev/ltms/fleet/peer/PeerLauncher.java

@@ -168,6 +168,16 @@ public interface PeerLauncher {
      * answer for a launcher with no role-pool concept of its own (e.g. a single {@code
      * HerdrPeerLauncher} adapter, which is never reached this way in production: {@code
      * CompositePeerLauncher} always fronts it and resolves roles itself).
+     *
+     * <p>fleetd #453: this default is deliberately <em>not</em> abstract — unlike {@link
+     * #spawn(SpawnRequest, PlacementDecision)} (fleetd #450), there is no live defect in inheriting
+     * it today, and the only current single-adapter implementer ({@code HerdrPeerLauncher}) is
+     * correct to do so. But it stays correct only as long as that holds: <strong>if a launcher ever
+     * routes more than one profile per role, it MUST override this method</strong>, or every role
+     * silently resolves to {@link #defaultProfile()} with no error and no log line. {@code
+     * HerdrPeerLauncher}'s own {@link #spawn(SpawnRequest, PlacementDecision)} javadoc names this
+     * method and {@link #place} explicitly as "unoverridden here" for exactly this reason — read
+     * that javadoc before adding role-pool routing to any {@code HerdrPeerLauncher} subclass.
      */
     default String defaultProfileFor(MemberRole role) {
         return defaultProfile();
@@ -228,6 +238,13 @@ public interface PeerLauncher {
      * placement condition — the right answer for a launcher with no pool or placement-policy
      * concept of its own, matching {@link #defaultProfileFor}'s own default.
      *
+     * <p>fleetd #453: same reasoning as {@link #defaultProfileFor}'s own #453 note — this default
+     * is deliberately not abstract (no live defect today, correct for the sole single-adapter
+     * implementer), but <strong>a launcher that ever routes more than one profile per role MUST
+     * override this method too</strong>, or placement silently ignores {@code role} for it. See
+     * {@code HerdrPeerLauncher.spawn(SpawnRequest, PlacementDecision)}'s javadoc, which names this
+     * method as "unoverridden here" and why that is correct only for a single-profile adapter.
+     *
      * @throws RuntimeException (implementation-specific, typically a placement exception) if no
      *                          candidate in {@code role}'s pool is currently placeable
      */