Merge CB-568: a dropped send reports the real cause

Injector.drop knew the precise cause (herdr agent_not_found) but the
sender was told only 'worker unreachable or stuck', so a lead could not
tell a dead pane from a stalled model.

TurnListener.onTurnFailed gains a reason, defaulting to the old one-arg
form. CompletionResolver prefers that reason, then the pane scrape, then
the old fixed text.

drop now fires onTurnFailed unconditionally. That is the substantive
fix: the sender blocks on the rendezvous waiter, not on the delivered
future, so failing delivered() alone never woke it and a queued send sat
until its timeout.

bridged/src/main/java/dev/ltms/bridged/Bridged.java

@@ -285,6 +285,12 @@ public final class Bridged {
                 completion.onTurnFailed(target);
                 sessions.onTurnFailed(target);
             }
+
+            @Override
+            public void onTurnFailed(String target, String reason) {
+                completion.onTurnFailed(target, reason);
+                sessions.onTurnFailed(target);
+            }
         };
         Injector injector = new Injector(agents, turnListener, deliverableTo(presence, leads),
                 presence::forget);

bridged/src/main/java/dev/ltms/bridged/inject/CompletionResolver.java

@@ -137,7 +137,13 @@ public final class CompletionResolver implements TurnListener {
     @Override
     public void onTurnFailed(String target) {
         InFlight turn = inFlight.get(target);
-        Thread.ofVirtual().name("turn-failed-" + target).start(() -> fail(target, turn));
+        Thread.ofVirtual().name("turn-failed-" + target).start(() -> fail(target, turn, null));
+    }
+
+    @Override
+    public void onTurnFailed(String target, String reason) {
+        InFlight turn = inFlight.get(target);
+        Thread.ofVirtual().name("turn-failed-" + target).start(() -> fail(target, turn, reason));
     }
 
     /** Synchronous resolve (the unit-testable core of {@link #onTurnComplete}). */
@@ -192,6 +198,11 @@ public final class CompletionResolver implements TurnListener {
 
     /** Synchronous fail (the unit-testable core of {@link #onTurnFailed}). */
     void fail(String target, InFlight turn) {
+        fail(target, turn, null);
+    }
+
+    /** Synchronous fail with an optional reason supplied by a dropped worker queue. */
+    void fail(String target, InFlight turn, String explicitReason) {
         // A never-delivered readiness failure has no in-flight record but still has a blocked send;
         // fall back to the currently-registered waiter (unambiguous — that send never completed, so
         // no next turn exists to confuse it with).
@@ -201,16 +212,18 @@ public final class CompletionResolver implements TurnListener {
             inFlight.remove(target, turn); // nobody blocked on this worker — nothing to fail
             return;
         }
-        String reason;
-        try {
-            reason = clip(agents.read(target, SCRAPE_SOURCE));
-        } catch (RuntimeException e) {
-            reason = "";
-        }
-        if (reason.isBlank()) {
-            // No screen to scrape — either the worker is stuck (CB-109) or gone (CB-110).
-            reason = "worker did not reply; its turn ended in an unrecoverable state "
-                    + "(worker unreachable or stuck)";
+        String reason = explicitReason;
+        if (reason == null || reason.isBlank()) {
+            try {
+                reason = clip(agents.read(target, SCRAPE_SOURCE));
+            } catch (RuntimeException e) {
+                reason = "";
+            }
+            if (reason.isBlank()) {
+                // No screen to scrape — either the worker is stuck (CB-109) or gone (CB-110).
+                reason = "worker did not reply; its turn ended in an unrecoverable state "
+                        + "(worker unreachable or stuck)";
+            }
         }
         if (rendezvous.resolveFailure(waiter, reason)) {
             inFlight.remove(target, turn);

bridged/src/main/java/dev/ltms/bridged/inject/Injector.java

@@ -410,8 +410,8 @@ public final class Injector {
         for (Pending p : pending) {
             p.delivered().completeExceptionally(cause);
         }
-        if (hadDeliveredTurn) {
-            turnListener.onTurnFailed(target);
-        }
+        // A queued send has no in-flight record, while a delivered turn does. CompletionResolver
+        // handles both forms and resolves its waiter at most once.
+        turnListener.onTurnFailed(target, cause.getMessage());
     }
 }

bridged/src/main/java/dev/ltms/bridged/inject/TurnListener.java

@@ -41,6 +41,14 @@ public interface TurnListener {
     default void onTurnFailed(String target) {
     }
 
+    /**
+     * As {@link #onTurnFailed(String)}, carrying the reason a worker became unreachable. The default
+     * keeps existing listeners working while allowing the completion resolver to report a useful cause.
+     */
+    default void onTurnFailed(String target, String reason) {
+        onTurnFailed(target);
+    }
+
     /**
      * A message was just delivered into {@code target}'s pane (CB-115). Fired so the completion
      * resolver can snapshot the pane's pre-turn content: a later {@link #onTurnComplete} whose