executor: internal dispatch queue; remove at-capacity rejection

Replace the at-capacity error return from Submit/SubmitResume with an
internal workCh/doneCh channel pair. A dispatch() goroutine blocks
waiting for a free slot and launches the worker goroutine, so tasks are
buffered up to 10x pool capacity instead of being rejected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 19 insertions, 11 deletions

diff --git a/internal/executor/executor_test.go b/internal/executor/executor_test.go
index 6d13873..414f852 100644
--- a/internal/executor/executor_test.go
+++ b/internal/executor/executor_test.go
@@ -217,26 +217,34 @@ func TestPool_Cancel_UnknownTask_ReturnsFalse(t *testing.T) {
 	}
 }
 
-func TestPool_AtCapacity(t *testing.T) {
+// TestPool_QueuedWhenAtCapacity verifies that Submit enqueues a task rather than
+// returning an error when the pool is at capacity. Both tasks should eventually complete.
+func TestPool_QueuedWhenAtCapacity(t *testing.T) {
 	store := testStore(t)
-	runner := &mockRunner{delay: time.Second}
+	runner := &mockRunner{delay: 100 * time.Millisecond}
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
 	pool := NewPool(1, runner, store, logger)
 
-	tk1 := makeTask("cap-1")
+	tk1 := makeTask("queue-1")
 	store.CreateTask(tk1)
-	pool.Submit(context.Background(), tk1)
+	if err := pool.Submit(context.Background(), tk1); err != nil {
+		t.Fatalf("first submit: %v", err)
+	}
 
-	// Pool is at capacity, second submit should fail.
-	time.Sleep(10 * time.Millisecond) // let goroutine start
-	tk2 := makeTask("cap-2")
+	// Second submit must succeed (queued) even though pool slot is taken.
+	tk2 := makeTask("queue-2")
 	store.CreateTask(tk2)
-	err := pool.Submit(context.Background(), tk2)
-	if err == nil {
-		t.Fatal("expected capacity error")
+	if err := pool.Submit(context.Background(), tk2); err != nil {
+		t.Fatalf("second submit: %v — expected task to be queued, not rejected", err)
 	}
 
-	<-pool.Results() // drain
+	// Both tasks must complete.
+	for i := 0; i < 2; i++ {
+		r := <-pool.Results()
+		if r.Err != nil {
+			t.Errorf("task %s error: %v", r.TaskID, r.Err)
+		}
+	}
 }
 
 // logPatherMockRunner is a mockRunner that also implements LogPather,