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>

videogen/interpolate.go

@@ -0,0 +1,82 @@
+package videogen
+
+import "context"
+
+// InterpolateRequest asks a frame-interpolation backend (RIFE-style) to
+// synthesize intermediate frames in an existing clip. Zero values mean
+// "backend default" (ADR-0020).
+type InterpolateRequest struct {
+	// Video is the clip to interpolate. Required.
+	Video Video
+
+	// Multi is the frame multiplier (2 or 4 on the reference backend);
+	// 0 = backend default (2).
+	Multi int
+
+	// SlowMo plays the synthesized frames at the SOURCE frame rate instead of
+	// multiplying it: Multi-times slow motion. The backend strips audio in
+	// this mode (time-stretched audio is noise).
+	SlowMo bool
+
+	// TargetFPS resamples the smoothed clip to a specific frame rate
+	// (e.g. 60); 0 = Multi times the source rate. Ignored in SlowMo mode.
+	TargetFPS int
+}
+
+// InterpolateOption mutates an InterpolateRequest before it is sent.
+type InterpolateOption func(*InterpolateRequest)
+
+// WithInterpolateMulti sets the frame multiplier.
+func WithInterpolateMulti(m int) InterpolateOption {
+	return func(r *InterpolateRequest) { r.Multi = m }
+}
+
+// WithSlowMo switches to slow-motion output.
+func WithSlowMo() InterpolateOption { return func(r *InterpolateRequest) { r.SlowMo = true } }
+
+// WithTargetFPS resamples the smoothed clip to a specific frame rate.
+func WithTargetFPS(fps int) InterpolateOption {
+	return func(r *InterpolateRequest) { r.TargetFPS = fps }
+}
+
+// Apply returns a copy of the request with all options applied.
+func (r InterpolateRequest) Apply(opts ...InterpolateOption) InterpolateRequest {
+	for _, opt := range opts {
+		opt(&r)
+	}
+	return r
+}
+
+// Interpolator synthesizes intermediate frames (fps boost or slow-mo). Its
+// own small interface rather than a method on Model: interpolators are not
+// generators — they bind to a different backend id entirely.
+type Interpolator interface {
+	// Interpolate returns the smoothed (or slowed) clip.
+	Interpolate(ctx context.Context, req InterpolateRequest, opts ...InterpolateOption) (*Result, error)
+}
+
+// InterpolatorModelOption configures an Interpolator at construction time.
+// Reserved for future per-model settings.
+type InterpolatorModelOption func(*InterpolatorModelConfig)
+
+// InterpolatorModelConfig carries per-model construction settings.
+type InterpolatorModelConfig struct{}
+
+// ApplyInterpolatorModelOptions folds options into a config.
+func ApplyInterpolatorModelOptions(opts []InterpolatorModelOption) InterpolatorModelConfig {
+	var cfg InterpolatorModelConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+	return cfg
+}
+
+// InterpolationProvider mints Interpolators bound to one backend.
+type InterpolationProvider interface {
+	// Name is the registry identifier for the provider.
+	Name() string
+
+	// InterpolatorModel returns an Interpolator bound to the given id (passed
+	// through to the backend verbatim; no catalog validation).
+	InterpolatorModel(id string, opts ...InterpolatorModelOption) (Interpolator, error)
+}