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/audio.go

@@ -77,7 +77,7 @@ func (m *speechModel) Speak(ctx context.Context, req audio.SpeechRequest, opts .
 // is a concrete audio type, else a mapping from the requested format, else
 // audio/mpeg (the OpenAI endpoint's default container is mp3).
 func speechMIME(contentType, format string) string {
-	if mt, _, err := mime.ParseMediaType(contentType); err == nil && strings.HasPrefix(mt, "audio/") {
+	if mt := mimeFromContentType(contentType, "audio/"); mt != "" {
 		return mt
 	}
 	switch strings.ToLower(strings.TrimSpace(format)) {
@@ -128,24 +128,13 @@ func (m *transcriptionModel) Transcribe(ctx context.Context, req audio.Transcrip
 	if _, err := fw.Write(req.Audio); err != nil {
 		return nil, fmt.Errorf("llama-swap: build transcription form: %w", err)
 	}
-	// Required fields always go on the wire (an empty model id should fail
-	// loudly upstream, not silently vanish); optional ones only when set.
-	fields := []struct {
-		key, value string
-		required   bool
-	}{
+	if err := writeFormFields(w, "build transcription form", []formField{
 		{"model", m.id, true},
 		{"response_format", "json", true},
 		{"language", req.Language, false},
 		{"prompt", req.Prompt, false},
-	}
-	for _, f := range fields {
-		if !f.required && f.value == "" {
-			continue
-		}
-		if err := w.WriteField(f.key, f.value); err != nil {
-			return nil, fmt.Errorf("llama-swap: build transcription form: %w", err)
-		}
+	}); err != nil {
+		return nil, err
 	}
 	if err := w.Close(); err != nil {
 		return nil, fmt.Errorf("llama-swap: build transcription form: %w", err)