Keep observer composition storage proportional to selected callbacks

docs/roadmap/remediation.md

@@ -140,7 +140,13 @@ statically. Do not add public allocation guarantees or exact-allocation tests.
 
 ## 3. Size Observer Storage For Selected Callbacks
 
-**Priority:** P3. **Status:** For consideration. **Plan stage:** 9.
+**Priority:** P3. **Status:** Complete. **Plan stage:** 9.
+
+**Completion:** Observer composition counts selected callbacks before allocating,
+sizes retained storage to that count, and handles zero/single selections without
+a fan-out closure. Sparse-selection ownership and duplicate-delivery tests pass.
+The complete offline race suite, maintained examples, vet, and diff checks passed
+with Go 1.27.3 after all remediations.
 
 **Location:** [`Compose`](../../internal/observation/compose.go).
 

internal/observation/compose.go

@@ -2,15 +2,26 @@ package observation
 
 // Compose copies and synchronously invokes non-nil observers in order.
 func Compose[T any, F ~func(T)](observers ...F) F {
-	selected := make([]F, 0, len(observers))
+	count := 0
+	var single F
+	for _, observer := range observers {
+		if observer != nil {
+			count++
+			single = observer
+		}
+	}
+	if count == 0 {
+		return nil
+	}
+	if count == 1 {
+		return single
+	}
+	selected := make([]F, 0, count)
 	for _, observer := range observers {
 		if observer != nil {
 			selected = append(selected, observer)
 		}
 	}
-	if len(selected) == 0 {
-		return nil
-	}
 	return func(event T) {
 		for _, observer := range selected {
 			observer(event)

observer_composition_contract_test.go

@@ -31,6 +31,31 @@ func TestComposeObserversCopiesAndOrders(t *testing.T) {
 	promptkit.ComposeObservers(func(promptkit.ExecutionEvent) { panic("panic") }, second)(promptkit.ExecutionEvent{})
 }
 
+func TestComposeObserversSparseSelection(t *testing.T) {
+	for _, selected := range []int{0, 1, 2} {
+		calls := 0
+		observers := make([]promptkit.ExecutionObserver, 1024)
+		callback := func(promptkit.ExecutionEvent) { calls++ }
+		for index := 0; index < selected; index++ {
+			observers[len(observers)-1-index] = callback
+		}
+		composed := promptkit.ComposeObservers(observers...)
+		for index := range observers {
+			observers[index] = func(promptkit.ExecutionEvent) { calls += 100 }
+		}
+		if selected == 0 {
+			if composed != nil {
+				t.Fatal("all-nil composition must be nil")
+			}
+			continue
+		}
+		composed(promptkit.ExecutionEvent{})
+		if calls != selected {
+			t.Fatalf("selected %d callbacks, got %d calls", selected, calls)
+		}
+	}
+}
+
 func TestCoordinateObserverCompositionAndNestedSelection(t *testing.T) {
 	engine := newSimpleEngine(t, "question", promptkit.WithLLMClient(&fakeLLMClient{response: &promptkit.GenerateResponse{Content: "answer"}}))
 	ctx := context.Background()