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/edit.go

@@ -12,6 +12,12 @@ type EditRequest struct {
 	// Init is the initial image the edit starts from. Required.
 	Init Image
 
+	// Mask restricts the edit to a region (inpainting): a single-channel or
+	// RGB image the same size as Init where WHITE pixels are repainted and
+	// BLACK pixels are kept. Empty = whole-image edit. Backends without mask
+	// support must reject a masked request rather than silently ignoring it.
+	Mask Image
+
 	// Strength is the denoising strength in [0,1] — how far the result may
 	// depart from Init (0 = return the input, 1 = ignore it); nil = backend
 	// default.
@@ -45,6 +51,9 @@ type EditRequest struct {
 // are applied to a copy of the request, so an EditRequest value can be reused.
 type EditOption func(*EditRequest)
 
+// WithEditMask restricts the edit to a region (white = repaint, black = keep).
+func WithEditMask(m Image) EditOption { return func(r *EditRequest) { r.Mask = m } }
+
 // WithEditStrength sets the denoising strength in [0,1].
 func WithEditStrength(s float64) EditOption { return func(r *EditRequest) { r.Strength = &s } }