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/upstream.go

@@ -0,0 +1,61 @@
+package llamaswap
+
+import (
+	"bytes"
+	"fmt"
+	"mime/multipart"
+	"strings"
+)
+
+// upstreamPath builds a path through llama-swap's generic /upstream/<model>/
+// passthrough, which pins the model (triggering the normal load/swap queue)
+// and forwards the remaining path to the upstream verbatim. This is how the
+// provider reaches upstreams whose native APIs carry no routable `model`
+// field (rembg, mediautils, WhisperX, ACE-Step, ...) without needing a
+// llama-swap route per endpoint (ADR-0020).
+//
+// Why reject rather than escape: same rationale as Unload — model ids
+// legitimately contain ":" but never path-structure characters, and escaping
+// would mask a config error instead of surfacing it.
+func upstreamPath(model, rest string) (string, error) {
+	if strings.TrimSpace(model) == "" {
+		return "", fmt.Errorf("llama-swap: upstream call requires a model id")
+	}
+	if strings.ContainsAny(model, "/?#") {
+		return "", fmt.Errorf("llama-swap: invalid model id %q for upstream call (contains a path separator)", model)
+	}
+	if !strings.HasPrefix(rest, "/") {
+		rest = "/" + rest
+	}
+	return "/upstream/" + model + rest, nil
+}
+
+// filePart is the single file entry of a media multipart form.
+type filePart struct {
+	field    string // form field name ("file", "audio_file", ...)
+	filename string // already sanitized
+	data     []byte
+}
+
+// buildMultipart assembles a one-file multipart body: the file part first,
+// then the given fields (optional fields skipped when empty, matching
+// writeFormFields). wrap labels errors. Returns the body and its content
+// type.
+func buildMultipart(wrap string, file filePart, fields []formField) (*bytes.Buffer, string, error) {
+	var buf bytes.Buffer
+	w := multipart.NewWriter(&buf)
+	fw, err := w.CreateFormFile(file.field, file.filename)
+	if err != nil {
+		return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+	}
+	if _, err := fw.Write(file.data); err != nil {
+		return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+	}
+	if err := writeFormFields(w, wrap, fields); err != nil {
+		return nil, "", err
+	}
+	if err := w.Close(); err != nil {
+		return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+	}
+	return &buf, w.FormDataContentType(), nil
+}