fix: address review — fail loud on non-video bodies, dedupe form plumbing, doc parity

- videoMIME no longer hard-falls-back to video/mp4: a 2xx body that is
  neither declared nor sniffable as video (JSON job envelope, HTML error
  page) is now an APIError instead of a 'successful' corrupt clip.
- Resolution rides the wire as width/height AND the OpenAI-style size
  string, so either upstream convention honors an explicit request.
- writeFormFields + mimeFromContentType shared helpers replace the
  copied multipart loop (audio.go/video.go) and Content-Type branch.
- ADR-0019 indexed in docs/adr/README.md; README gains the videogen
  section + support-matrix mention (docs-parity rule).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

provider/llamaswap/llamaswap.go

@@ -30,6 +30,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"mime"
+	"mime/multipart"
 	"net/http"
 	"strings"
 
@@ -280,3 +282,34 @@ func (p *Provider) apiError(resp *http.Response, model string) error {
 	e.Message = strings.TrimSpace(string(body))
 	return e
 }
+
+// formField is one multipart form entry for the media endpoints. Required
+// fields always go on the wire (an empty model id should fail loudly
+// upstream, not silently vanish); optional ones only when set.
+type formField struct {
+	key, value string
+	required   bool
+}
+
+// writeFormFields appends fields to a multipart writer, skipping unset
+// optional entries. wrap labels errors ("build transcription form", ...).
+func writeFormFields(w *multipart.Writer, wrap string, fields []formField) error {
+	for _, f := range fields {
+		if !f.required && f.value == "" {
+			continue
+		}
+		if err := w.WriteField(f.key, f.value); err != nil {
+			return fmt.Errorf("llama-swap: %s: %w", wrap, err)
+		}
+	}
+	return nil
+}
+
+// mimeFromContentType returns the parsed media type when it matches prefix
+// ("audio/", "video/"), else "".
+func mimeFromContentType(contentType, prefix string) string {
+	if mt, _, err := mime.ParseMediaType(contentType); err == nil && strings.HasPrefix(mt, prefix) {
+		return mt
+	}
+	return ""
+}