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>

imagegen/upscale.go

@@ -0,0 +1,62 @@
+package imagegen
+
+import "context"
+
+// UpscaleRequest asks a super-resolution backend to enlarge an image. Zero
+// values mean "backend default", mirroring EditRequest (ADR-0020).
+type UpscaleRequest struct {
+	// Image is the image to upscale. Required.
+	Image Image
+
+	// Scale is the enlargement factor (2 or 4 on the reference backend);
+	// 0 = backend default.
+	Scale int
+}
+
+// UpscaleOption mutates an UpscaleRequest before it is sent.
+type UpscaleOption func(*UpscaleRequest)
+
+// WithUpscaleScale sets the enlargement factor.
+func WithUpscaleScale(s int) UpscaleOption { return func(r *UpscaleRequest) { r.Scale = s } }
+
+// Apply returns a copy of the request with all options applied.
+func (r UpscaleRequest) Apply(opts ...UpscaleOption) UpscaleRequest {
+	for _, opt := range opts {
+		opt(&r)
+	}
+	return r
+}
+
+// Upscaler enlarges images with a super-resolution model. It is its own
+// small interface (not a method on Model) because upscalers are not
+// diffusion models — they bind to a different backend id entirely.
+type Upscaler interface {
+	// Upscale returns the enlarged image (a one-image Result).
+	Upscale(ctx context.Context, req UpscaleRequest, opts ...UpscaleOption) (*Result, error)
+}
+
+// UpscaleModelOption configures an Upscaler at construction time. Reserved
+// for future per-model settings (mirrors ModelOption).
+type UpscaleModelOption func(*UpscaleModelConfig)
+
+// UpscaleModelConfig carries per-model construction settings.
+type UpscaleModelConfig struct{}
+
+// ApplyUpscaleModelOptions folds options into a config.
+func ApplyUpscaleModelOptions(opts []UpscaleModelOption) UpscaleModelConfig {
+	var cfg UpscaleModelConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+	return cfg
+}
+
+// UpscaleProvider mints Upscalers bound to one backend.
+type UpscaleProvider interface {
+	// Name is the registry identifier for the provider.
+	Name() string
+
+	// UpscaleModel returns an Upscaler bound to the given id (passed through
+	// to the backend verbatim; no catalog validation).
+	UpscaleModel(id string, opts ...UpscaleModelOption) (Upscaler, error)
+}