feat(llm): make runs resilient and reproducible on slow backends

Retry with exponential backoff and jitter, and honour Retry-After. Retry empty-choice responses and HTTP 408.
Add max_retries, retry_timeouts, model_timeouts and model_concurrency settings for slow local models.
Add opt-in SSE streaming with an idle timeout. A hung server fails early, and a slow but active generation continues.
Send an optional seed and record the backend fingerprint of each answer.
Stamp saved artifacts with run metadata and write them atomically. Warn in compare when two runs differ.
Save partial results on Ctrl-C. Turn an evaluator panic into an error row.
Reject duplicate models and non-http endpoints in the config.

internal/runner/runner_test.go

@@ -410,3 +410,89 @@ func TestRunner_Run_RepeatSamplesEachCombination(t *testing.T) {
 		}
 	}
 }
+
+func TestRunner_Run_PassesSeedAndRecordsFingerprint(t *testing.T) {
+	var gotSeed *int
+	client := &mockClient{respond: func(req llm.Request) (llm.Response, error) {
+		gotSeed = req.Seed
+		return llm.Response{Text: "OK", Fingerprint: "fp-1", ServedModel: "served"}, nil
+	}}
+	seed := 7
+	r := New(client, Config{Concurrency: 1, MaxTokensDefault: 100, Seed: &seed})
+	res := r.Run(context.Background(), []string{"m"}, []testkit.Test{{ID: "t", Category: "c", Prompt: "p", Eval: echoEval()}})
+	if gotSeed == nil || *gotSeed != 7 {
+		t.Errorf("request seed = %v, want 7", gotSeed)
+	}
+	if res[0].Fingerprint != "fp-1" || res[0].ServedModel != "served" {
+		t.Errorf("fingerprint/model = %q/%q, want fp-1/served", res[0].Fingerprint, res[0].ServedModel)
+	}
+}
+
+func TestRunner_Run_EvaluatorPanicBecomesErrorResult(t *testing.T) {
+	boom := eval.EvaluatorFunc(func(context.Context, string) eval.Score { panic("boom") })
+	r := New(&mockClient{}, Config{Concurrency: 2, MaxTokensDefault: 100})
+	res := r.Run(context.Background(), []string{"m"}, []testkit.Test{
+		{ID: "bad", Category: "c", Prompt: "p", Eval: boom},
+		{ID: "good", Category: "c", Prompt: "p", Eval: echoEval()},
+	})
+	if res[0].Err == nil || !strings.Contains(res[0].Err.Error(), "boom") {
+		t.Errorf("panicking test Err = %v, want panic error", res[0].Err)
+	}
+	if res[1].Err != nil || res[1].Score.Value != 1 {
+		t.Errorf("sibling test = %+v, want unaffected pass", res[1])
+	}
+}
+
+func TestRunner_Run_CancelledContextSkipsCalls(t *testing.T) {
+	var calls int32
+	client := &mockClient{respond: func(llm.Request) (llm.Response, error) {
+		atomic.AddInt32(&calls, 1)
+		return llm.Response{Text: "OK"}, nil
+	}}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	r := New(client, Config{Concurrency: 2, MaxTokensDefault: 100})
+	res := r.Run(ctx, []string{"m"}, []testkit.Test{{ID: "t", Category: "c", Prompt: "p", Eval: echoEval()}})
+	if atomic.LoadInt32(&calls) != 0 {
+		t.Errorf("client called %d times after cancel, want 0", calls)
+	}
+	if !errors.Is(res[0].Err, context.Canceled) {
+		t.Errorf("Err = %v, want context.Canceled", res[0].Err)
+	}
+}
+
+func TestRunner_Run_ModelConcurrencyCapDoesNotStarveOthers(t *testing.T) {
+	var slowIn, slowMax int32
+	fastDone := make(chan struct{}, 8)
+	client := &mockClient{respond: func(req llm.Request) (llm.Response, error) {
+		if req.Model == "slow" {
+			cur := atomic.AddInt32(&slowIn, 1)
+			for {
+				m := atomic.LoadInt32(&slowMax)
+				if cur <= m || atomic.CompareAndSwapInt32(&slowMax, m, cur) {
+					break
+				}
+			}
+			time.Sleep(30 * time.Millisecond)
+			atomic.AddInt32(&slowIn, -1)
+		} else {
+			fastDone <- struct{}{}
+		}
+		return llm.Response{Text: "OK"}, nil
+	}}
+	r := New(client, Config{Concurrency: 2, MaxTokensDefault: 100, ModelConcurrency: map[string]int{"slow": 1}})
+	tests := make([]testkit.Test, 6)
+	for i := range tests {
+		tests[i] = testkit.Test{ID: string(rune('a' + i)), Category: "c", Prompt: "p", Eval: echoEval()}
+	}
+	res := r.Run(context.Background(), []string{"slow", "fast"}, tests)
+	if len(res) != 12 {
+		t.Fatalf("results = %d, want 12", len(res))
+	}
+	if got := atomic.LoadInt32(&slowMax); got != 1 {
+		t.Errorf("slow model max in-flight = %d, want 1", got)
+	}
+	if len(fastDone) != 6 {
+		t.Errorf("fast calls = %d, want 6", len(fastDone))
+	}
+}