Exercise queued and failed material intake through HTTP

Why: The stage-6 browser proof must cover the real queued and failed-start user states.

Evidence: Two Playwright paths failed before fixture support, then passed; full Backend vet and Go tests passed.

Impact: Adds only task-owned mechanism fixture behavior under the live test tag.

Refs: U3 r0.5 independent review finding

internal/server/work_checkpoint_eval_test.go

@@ -108,6 +108,9 @@ func evalMechanismStream(ctx context.Context, model string, msgs []agentllm.Mess
 	content := "已收到材料，正在后台整理；请结合阶段发现继续工作。"
 	var calls []agentllm.ToolCall
 	if checkpoint {
+		if toolReplies == 0 && strings.Contains(sourceText, "[fixture:queue-hold]") {
+			time.Sleep(8 * time.Second)
+		}
 		count := toolReplies
 		matches := mechanismCheckpointSources(sourceText)
 		matches = filterCheckpointSources(matches)
@@ -280,7 +283,44 @@ func TestServeWorkCheckpointEval(t *testing.T) {
 	if e = os.WriteFile(manifest, raw, 0600); e != nil {
 		t.Fatal(e)
 	}
-	httpServer := &http.Server{Handler: server.Handler()}
+	baseHandler := server.Handler()
+	var handler http.Handler = baseHandler
+	if mode == "mechanism" {
+		handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/__fixture/material-start-failure" {
+				baseHandler.ServeHTTP(w, r)
+				return
+			}
+			if r.Method != http.MethodPost {
+				w.WriteHeader(http.StatusMethodNotAllowed)
+				return
+			}
+			var request struct {
+				Enabled bool `json:"enabled"`
+			}
+			if err := json.NewDecoder(r.Body).Decode(&request); err != nil {
+				http.Error(w, "invalid fixture request", http.StatusBadRequest)
+				return
+			}
+			db, err := sql.Open("sqlite", filepath.Join(dir, "agent-system.db"))
+			if err != nil {
+				http.Error(w, "fixture storage unavailable", http.StatusInternalServerError)
+				return
+			}
+			defer db.Close()
+			statement := `DROP TRIGGER IF EXISTS fixture_fail_material_claim`
+			if request.Enabled {
+				statement = `CREATE TRIGGER IF NOT EXISTS fixture_fail_material_claim BEFORE INSERT ON conversation_executions
+					WHEN NEW.conversation_id LIKE 'task:%' BEGIN SELECT RAISE(FAIL, 'fixture material claim failure'); END`
+			}
+			if _, err := db.Exec(statement); err != nil {
+				http.Error(w, "fixture storage fault unavailable", http.StatusInternalServerError)
+				return
+			}
+			w.WriteHeader(http.StatusNoContent)
+		})
+	}
+	httpServer := &http.Server{Handler: handler}
 	done := make(chan error, 1)
 	go func() { done <- httpServer.Serve(listener) }()
 	stop := make(chan os.Signal, 1)