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 <[email protected]>

provider/llamaswap/image.go

@@ -128,6 +128,11 @@ type img2imgRequest struct {
 	txt2imgRequest
 	InitImages        []string `json:"init_images"`
 	DenoisingStrength *float64 `json:"denoising_strength,omitempty"`
+	// Mask enables inpainting: base64 image, white = repaint, black = keep
+	// (sd-server also accepts a data URL; plain base64 keeps symmetry with
+	// init_images). sd-server has no mask_blur/inpaint_full_res — callers
+	// wanting soft edges pre-feather the mask.
+	Mask string `json:"mask,omitempty"`
 }
 
 // Edit implements imagegen.Editor via POST {base}/sdapi/v1/img2img.
@@ -148,6 +153,9 @@ func (m *imageModel) Edit(ctx context.Context, req imagegen.EditRequest, opts ..
 		InitImages:        []string{base64.StdEncoding.EncodeToString(req.Init.Data)},
 		DenoisingStrength: req.Strength,
 	}
+	if len(req.Mask.Data) > 0 {
+		wire.Mask = base64.StdEncoding.EncodeToString(req.Mask.Data)
+	}
 
 	var resp txt2imgResponse
 	if err := m.p.doJSON(ctx, http.MethodPost, "/sdapi/v1/img2img", m.id, &wire, &resp); err != nil {