feat(imagegen): optional per-request generation settings

Add Steps, CFGScale, NegativePrompt, Sampler, Seed to imagegen.Request
(pointer/empty = leave the backend's per-model default), with mirror
options, and forward them in the llamaswap wire payload as the
stable-diffusion.cpp fields (steps/cfg_scale/negative_prompt/
sample_method/seed). Unset fields are omitted so sd-server keeps its
baked defaults.

Lets callers (e.g. mort drawbots) override only what they explicitly set.

provider/llamaswap/image.go

@@ -27,14 +27,23 @@ type imageModel struct {
 	id string
 }
 
-// imageRequest is the OpenAI /v1/images/generations request shape. We always
-// request b64_json so the bytes come back inline (no second fetch).
+// imageRequest is the OpenAI /v1/images/generations request shape, plus the
+// stable-diffusion.cpp extras llama-swap forwards to sd-server. We always
+// request b64_json so the bytes come back inline (no second fetch). The
+// optional fields are pointers/omitempty so an unset value is omitted entirely
+// and sd-server falls back to the model's own default (a field name a given
+// sd-server build doesn't recognize is simply ignored — harmless).
 type imageRequest struct {
-	Model          string `json:"model"`
-	Prompt         string `json:"prompt"`
-	N              int    `json:"n,omitempty"`
-	Size           string `json:"size,omitempty"`
-	ResponseFormat string `json:"response_format"`
+	Model          string   `json:"model"`
+	Prompt         string   `json:"prompt"`
+	N              int      `json:"n,omitempty"`
+	Size           string   `json:"size,omitempty"`
+	ResponseFormat string   `json:"response_format"`
+	Steps          *int     `json:"steps,omitempty"`
+	CFGScale       *float64 `json:"cfg_scale,omitempty"`
+	NegativePrompt string   `json:"negative_prompt,omitempty"`
+	SampleMethod   string   `json:"sample_method,omitempty"`
+	Seed           *int64   `json:"seed,omitempty"`
 }
 
 type imageResponse struct {
@@ -61,6 +70,11 @@ func (m *imageModel) Generate(ctx context.Context, req imagegen.Request, opts ..
 		N:              req.N,
 		Size:           req.Size,
 		ResponseFormat: "b64_json",
+		Steps:          req.Steps,
+		CFGScale:       req.CFGScale,
+		NegativePrompt: req.NegativePrompt,
+		SampleMethod:   req.Sampler,
+		Seed:           req.Seed,
 	}
 
 	var resp imageResponse

provider/llamaswap/llamaswap_test.go

@@ -201,6 +201,48 @@ func TestImageGenerate(t *testing.T) {
 	}
 }
 
+func TestImageGenerateSettings(t *testing.T) {
+	var gotBody map[string]any
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_ = json.NewDecoder(r.Body).Decode(&gotBody)
+		_, _ = w.Write([]byte(`{"created":1,"data":[{"b64_json":"` + onePixelPNG + `"}]}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	im, _ := p.ImageModel("sd")
+
+	// Unset overrides must be omitted entirely so sd-server keeps its own
+	// per-model defaults.
+	if _, err := im.Generate(context.Background(), imagegen.Request{Prompt: "x"}); err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+	for _, k := range []string{"steps", "cfg_scale", "negative_prompt", "sample_method", "seed"} {
+		if v, ok := gotBody[k]; ok {
+			t.Errorf("unset request sent %q = %v, want omitted", k, v)
+		}
+	}
+
+	// Set overrides are forwarded with the sd-server-friendly field names.
+	gotBody = nil
+	_, err := im.Generate(context.Background(), imagegen.Request{Prompt: "x"},
+		imagegen.WithSteps(8),
+		imagegen.WithCFGScale(3.5),
+		imagegen.WithNegativePrompt("blurry"),
+		imagegen.WithSampler("euler"),
+		imagegen.WithSeed(42),
+	)
+	if err != nil {
+		t.Fatalf("Generate: %v", err)
+	}
+	want := map[string]any{"steps": float64(8), "cfg_scale": 3.5, "negative_prompt": "blurry", "sample_method": "euler", "seed": float64(42)}
+	for k, w := range want {
+		if gotBody[k] != w {
+			t.Errorf("%s = %v, want %v", k, gotBody[k], w)
+		}
+	}
+}
+
 func TestImageGenerateEmptyPrompt(t *testing.T) {
 	p := New(WithBaseURL("http://example.invalid"))
 	im, _ := p.ImageModel("sd")