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/background.go

@@ -0,0 +1,76 @@
+package imagegen
+
+import "context"
+
+// BackgroundRemovalRequest asks a matting/segmentation backend to cut the
+// subject out of an image. Zero values mean "backend default" (ADR-0020).
+type BackgroundRemovalRequest struct {
+	// Image is the image to process. Required.
+	Image Image
+
+	// Net selects the remover's internal network where the backend offers
+	// several (rembg: "u2net", "birefnet-general", "isnet-general-use", ...);
+	// "" = backend default. This is NOT the provider model id — that is fixed
+	// when the BackgroundRemover is minted.
+	Net string
+
+	// OnlyMask returns the black/white segmentation mask instead of the
+	// cutout — white where the subject is. Useful as an inpainting mask
+	// (EditRequest.Mask semantics after inversion: the mask marks the
+	// FOREGROUND, an inpaint mask marks the region to REPAINT).
+	OnlyMask bool
+}
+
+// BackgroundRemovalOption mutates a BackgroundRemovalRequest before it is sent.
+type BackgroundRemovalOption func(*BackgroundRemovalRequest)
+
+// WithBackgroundNet selects the remover's internal network.
+func WithBackgroundNet(n string) BackgroundRemovalOption {
+	return func(r *BackgroundRemovalRequest) { r.Net = n }
+}
+
+// WithOnlyMask requests the segmentation mask instead of the cutout.
+func WithOnlyMask() BackgroundRemovalOption {
+	return func(r *BackgroundRemovalRequest) { r.OnlyMask = true }
+}
+
+// Apply returns a copy of the request with all options applied.
+func (r BackgroundRemovalRequest) Apply(opts ...BackgroundRemovalOption) BackgroundRemovalRequest {
+	for _, opt := range opts {
+		opt(&r)
+	}
+	return r
+}
+
+// BackgroundRemover cuts subjects out of images (RGBA PNG with transparent
+// background, or the mask alone with OnlyMask).
+type BackgroundRemover interface {
+	// RemoveBackground returns the cutout (or mask) as a one-image Result.
+	RemoveBackground(ctx context.Context, req BackgroundRemovalRequest, opts ...BackgroundRemovalOption) (*Result, error)
+}
+
+// BackgroundRemovalModelOption configures a BackgroundRemover at construction
+// time. Reserved for future per-model settings.
+type BackgroundRemovalModelOption func(*BackgroundRemovalModelConfig)
+
+// BackgroundRemovalModelConfig carries per-model construction settings.
+type BackgroundRemovalModelConfig struct{}
+
+// ApplyBackgroundRemovalModelOptions folds options into a config.
+func ApplyBackgroundRemovalModelOptions(opts []BackgroundRemovalModelOption) BackgroundRemovalModelConfig {
+	var cfg BackgroundRemovalModelConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+	return cfg
+}
+
+// BackgroundRemovalProvider mints BackgroundRemovers bound to one backend.
+type BackgroundRemovalProvider interface {
+	// Name is the registry identifier for the provider.
+	Name() string
+
+	// BackgroundRemovalModel returns a BackgroundRemover bound to the given
+	// id (passed through to the backend verbatim; no catalog validation).
+	BackgroundRemovalModel(id string, opts ...BackgroundRemovalModelOption) (BackgroundRemover, error)
+}