Merge #456: document the override obligation on PeerLauncher.defaultProfileFor/place (fleetd #453)

Doc-only, one file, 17 added lines. Verified by me on a local merge of 0788d84
onto 2af13ab (merge commit 5b46538):

- mvn -B clean test: Tests run: 1578, Failures: 0, Errors: 0, Skipped: 0. BUILD SUCCESS, rc=0.
- javadoc reference lint (mvn javadoc:javadoc -Ddoclint=reference): the merge has 5
  "reference not found" errors; origin/main at 2af13ab has the same 5, in the same 5
  untouched files. So this PR adds no broken link. Both runs rc=1 for that pre-existing
  reason, which is now ticket #459.
- The quoted phrase is real: "are unoverridden here and just wrap {@link #defaultProfile()}"
  is at member/HerdrPeerLauncher.java:604. place() is not overloaded, so {@link #place}
  is unambiguous.

One inaccuracy I am fixing in a follow-up commit rather than sending the PR back: the first
new paragraph writes "HerdrPeerLauncher's own {@link #spawn(SpawnRequest, PlacementDecision)}
javadoc", but that link resolves to PeerLauncher's own abstract declaration, not to
HerdrPeerLauncher's override. The second paragraph already gets this right with plain
{@code HerdrPeerLauncher.spawn(...)}.

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
      */