feat: media expansion surfaces — edit mask, upscale, background removal, interpolation, diarization, meshgen (ADR-0020)

- imagegen.EditRequest.Mask -> sd-server img2img inpainting (white=repaint)
- imagegen.Upscaler + BackgroundRemover, videogen.Interpolator,
  audio.DiarizationModel: new optional provider-minted surfaces
- NEW meshgen leaf package (image->3D, glb/stl/obj)
- provider/llamaswap: all five via the /upstream/<model>/<path> passthrough
  (upstreamPath helper, shared one-file multipart builder); binary success
  bodies validated (non-image, non-video, JSON-mesh rejection); diarization
  pins output=json (vtt/srt drop speaker labels)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

provider/llamaswap/edit_test.go

@@ -108,3 +108,47 @@ func TestImageEditValidation(t *testing.T) {
 		t.Errorf("out-of-range strength: err = %v, want ErrUnsupported", err)
 	}
 }
+
+func TestImageEditWithMask(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(`{"images":["` + onePixelPNG + `"]}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	im, _ := p.ImageModel("sd")
+	ed := im.(imagegen.Editor)
+
+	mask := imagegen.Image{MIME: "image/png", Data: []byte{0xDE, 0xAD}}
+	if _, err := ed.Edit(context.Background(),
+		imagegen.EditRequest{Prompt: "replace the sky", Init: editInit(t)},
+		imagegen.WithEditMask(mask),
+	); err != nil {
+		t.Fatalf("Edit: %v", err)
+	}
+	if got := gotBody["mask"]; got != base64.StdEncoding.EncodeToString(mask.Data) {
+		t.Errorf("mask = %v, want the b64 mask", got)
+	}
+}
+
+func TestImageEditWithoutMaskOmitsField(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(`{"images":["` + onePixelPNG + `"]}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	im, _ := p.ImageModel("sd")
+	ed := im.(imagegen.Editor)
+	if _, err := ed.Edit(context.Background(),
+		imagegen.EditRequest{Prompt: "p", Init: editInit(t)}); err != nil {
+		t.Fatalf("Edit: %v", err)
+	}
+	if _, ok := gotBody["mask"]; ok {
+		t.Error("mask field sent for unmasked edit; want omitted")
+	}
+}