fix(llamaswap): use A1111 /sdapi/v1/txt2img so seed is honored

The OpenAI /v1/images/generations endpoint ignores `seed` on our
stable-diffusion.cpp build — every render of a given prompt comes back
byte-identical, so a drawbot batch of N collapsed to one image. Switch the
image provider to sd-server's A1111 /sdapi/v1/txt2img endpoint, which honors
`seed` (verified live: distinct seeds -> distinct images on SDXL and
Qwen-Image). Size is split into width/height; llama-swap still routes by the
`model` field. Tests + ADR-0016 updated.

provider/llamaswap/llamaswap_test.go

@@ -166,11 +166,11 @@ func TestRunningRaw(t *testing.T) {
 func TestImageGenerate(t *testing.T) {
 	var gotBody map[string]any
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		if r.URL.Path != "/v1/images/generations" {
+		if r.URL.Path != "/sdapi/v1/txt2img" {
 			t.Errorf("path = %q", r.URL.Path)
 		}
 		_ = json.NewDecoder(r.Body).Decode(&gotBody)
-		_, _ = w.Write([]byte(`{"created":1,"data":[{"b64_json":"` + onePixelPNG + `"}]}`))
+		_, _ = w.Write([]byte(`{"images":["` + onePixelPNG + `"]}`))
 	}))
 	defer srv.Close()
 
@@ -192,12 +192,9 @@ func TestImageGenerate(t *testing.T) {
 	if len(res.Images[0].Data) == 0 {
 		t.Error("decoded image has no bytes")
 	}
-	// response_format must be forced to b64_json, and options applied.
-	if gotBody["response_format"] != "b64_json" {
-		t.Errorf("response_format = %v, want b64_json", gotBody["response_format"])
-	}
-	if gotBody["size"] != "512x512" {
-		t.Errorf("size = %v, want 512x512", gotBody["size"])
+	// Size is split into width/height ints for the A1111 endpoint.
+	if gotBody["width"] != float64(512) || gotBody["height"] != float64(512) {
+		t.Errorf("width/height = %v/%v, want 512/512", gotBody["width"], gotBody["height"])
 	}
 }
 
@@ -205,7 +202,7 @@ 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 + `"}]}`))
+		_, _ = w.Write([]byte(`{"images":["` + onePixelPNG + `"]}`))
 	}))
 	defer srv.Close()