fleetd #450: make PeerLauncher.spawn(SpawnRequest, PlacementDecision) abstract

The default re-entered the single-argument spawn(SpawnRequest), which re-runs
checks that can refuse the profile place() just chose (#444's window). Only
CompositePeerLauncher overrode it; a future placement-doing launcher could
have inherited the wrong body silently.

Give every current implementer an explicit override, chosen by what it does:
- HerdrPeerLauncher (base of ClaudeCodeLauncher/OpenCodeLauncher, neither of
  which overrides spawn(req) or place()) does no placement filtering of its
  own, so it gets the re-entering form.
- CompositePeerLauncher's routing-form override is untouched.
- 5 test-fake PeerLauncher implementers (SessionManagerTest, FleetdBackendErrorSinkTest)
  get overrides matching their existing spawn(SpawnRequest) shape: delegating
  wrappers delegate, unreachable stubs throw, the single-profile fake re-enters.

ConfigRef does not implement PeerLauncher at all (confirmed in this tree at
822327e) despite the ticket listing it as a src/main implementer.

fleetd/src/main/java/dev/ltms/fleet/member/HerdrPeerLauncher.java

@@ -17,6 +17,7 @@ import dev.ltms.fleet.peer.PeerHandle;
 import dev.ltms.fleet.peer.PeerLauncher;
 import dev.ltms.fleet.peer.PeerUnreachableException;
 import dev.ltms.fleet.peer.SpawnRequest;
+import dev.ltms.fleet.placement.PlacementDecision;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -594,6 +595,23 @@ public abstract class HerdrPeerLauncher implements PeerLauncher {
                 req.sessionName(), spawned.agentSessionId(), spawned.receipt());
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * <p>fleetd #450: re-enters {@link #spawn(SpawnRequest)} with {@code decision}'s profile named
+     * explicitly. This is the re-entering form the interface javadoc describes for a launcher with
+     * no placement concept of its own — an instance of this class spawns a single adapter's own
+     * profile set by explicit name only ({@link #place}/{@link #defaultProfileFor} are unoverridden
+     * here and just wrap {@link #defaultProfile()}); it does no quarantine/cool-off/maxLoad/model-off
+     * filtering of its own to re-apply. That filtering lives one layer up, in {@code
+     * CompositePeerLauncher}, which is the launcher that routes across more than one profile and
+     * therefore overrides this method with the routing form instead.
+     */
+    @Override
+    public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+        return spawn(req.withProfile(decision.profile()));
+    }
+
     /** The herdr daemon that owns this launcher's pane coordinates. */
     public HerdrClient herdr() {
         return agents.herdr();

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

@@ -253,26 +253,27 @@ public interface PeerLauncher {
      * matching profile; passing a request that names a <em>different</em>, explicit profile than
      * the decision it is paired with is a caller bug this method does not attempt to detect.
      *
-     * <p>Default implementation for a launcher with no placement concept of its own: delegates to
-     * {@link #spawn(SpawnRequest)} with the decision's profile named explicitly — its only spawn
-     * contract, since there is no separate routing path to honor. This default is correct ONLY for
-     * a launcher that spawns a single profile of its own (e.g. {@code HerdrPeerLauncher}), where
-     * the explicit-profile branch it re-enters and the routing branch {@link #place} would have
-     * used are the same thing. <strong>A launcher that routes across more than one profile — the
-     * way {@code CompositePeerLauncher} routes across every configured adapter — MUST override
-     * this method instead of inheriting this default.</strong> Re-entering {@link
-     * #spawn(SpawnRequest)} re-applies that single-argument method's explicit-profile checks
-     * ({@code enforceNotQuarantined}, {@code enforceNotCoolingOff}, {@code enforceMaxLoad}, {@code
-     * enforceModelEnabled} in {@code CompositePeerLauncher}), which can refuse the very profile
-     * {@link #place} just chose, if the underlying placement state moved in the window between the
-     * {@link #place} call and this one — the exact window this method and {@link PlacementDecision}
-     * exist to close (fleetd #444).
+     * <p>No default implementation (fleetd #450): the two correct bodies disagree on purpose, so an
+     * implementer must choose one rather than silently inherit whichever this interface happened to
+     * provide. An implementer with no placement concept of its own — spawns a single profile, e.g.
+     * {@code HerdrPeerLauncher} — should delegate to {@link #spawn(SpawnRequest)} with the decision's
+     * profile named explicitly, since there is no separate routing path to honor there: the
+     * explicit-profile branch it re-enters and the routing branch {@link #place} would have used are
+     * the same thing. <strong>A launcher that routes across more than one profile — the way {@code
+     * CompositePeerLauncher} routes across every configured adapter — MUST NOT re-enter {@link
+     * #spawn(SpawnRequest)}.</strong> Doing so re-applies that single-argument method's
+     * explicit-profile checks ({@code enforceNotQuarantined}, {@code enforceNotCoolingOff}, {@code
+     * enforceMaxLoad}, {@code enforceModelEnabled} in {@code CompositePeerLauncher}), which can
+     * refuse the very profile {@link #place} just chose, if the underlying placement state moved in
+     * the window between the {@link #place} call and this one — the exact window this method and
+     * {@link PlacementDecision} exist to close (fleetd #444). Before #450 this was a {@code default}
+     * method that only {@code CompositePeerLauncher} overrode; a future placement-doing launcher
+     * could have inherited the re-entering body silently and never known. Making it abstract turns
+     * that silent inheritance into a compile error.
      *
      * @throws IllegalArgumentException if the decision names an unknown profile
      */
-    default PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
-        return spawn(req.withProfile(decision.profile()));
-    }
+    PeerHandle spawn(SpawnRequest req, PlacementDecision decision);
 
     /**
      * Resolve the effective working directory for a spawn {@code req} without actually spawning.

fleetd/src/test/java/dev/ltms/fleet/FleetdBackendErrorSinkTest.java

@@ -20,6 +20,7 @@ import dev.ltms.fleet.peer.PeerLauncher;
 import dev.ltms.fleet.peer.SpawnRequest;
 import dev.ltms.fleet.placement.BackendOutagePolicy;
 import dev.ltms.fleet.placement.BackendQuarantine;
+import dev.ltms.fleet.placement.PlacementDecision;
 import dev.ltms.fleet.placement.PlacementPolicies;
 import dev.ltms.fleet.session.MemberSession;
 import dev.ltms.fleet.session.SessionManager;
@@ -123,6 +124,11 @@ class FleetdBackendErrorSinkTest {
             throw new UnsupportedOperationException("not reachable — this test never acquires a session");
         }
 
+        @Override
+        public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+            throw new UnsupportedOperationException("not reachable — this test never acquires a session");
+        }
+
         @Override
         public Set<String> profiles() {
             return Set.of();

fleetd/src/test/java/dev/ltms/fleet/session/SessionManagerTest.java

@@ -23,6 +23,7 @@ import dev.ltms.fleet.peer.PeerLauncher;
 import dev.ltms.fleet.peer.PeerUnreachableException;
 import dev.ltms.fleet.peer.SpawnRequest;
 import dev.ltms.fleet.placement.BackendQuarantine;
+import dev.ltms.fleet.placement.PlacementDecision;
 import dev.ltms.fleet.placement.PlacementPolicies;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
@@ -1036,6 +1037,11 @@ class SessionManagerTest {
             return delegate.spawn(req);
         }
 
+        @Override
+        public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+            return delegate.spawn(req, decision);
+        }
+
         @Override
         public Set<String> profiles() {
             return delegate.profiles();
@@ -1597,6 +1603,11 @@ class SessionManagerTest {
             throw new UnsupportedOperationException("not reachable — the capability check refuses first");
         }
 
+        @Override
+        public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+            throw new UnsupportedOperationException("not reachable — the capability check refuses first");
+        }
+
         @Override
         public Set<String> profiles() {
             return Set.of("stub-profile");
@@ -1661,6 +1672,11 @@ class SessionManagerTest {
             return delegate.spawn(req);
         }
 
+        @Override
+        public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+            return delegate.spawn(req, decision);
+        }
+
         @Override
         public Set<String> profiles() {
             return delegate.profiles();
@@ -1867,6 +1883,11 @@ class SessionManagerTest {
             return handle;
         }
 
+        @Override
+        public PeerHandle spawn(SpawnRequest req, PlacementDecision decision) {
+            return spawn(req.withProfile(decision.profile()));
+        }
+
         @Override
         public Set<String> profiles() {
             return Set.of("lazy");