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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

docs/adr/README.md

@@ -22,3 +22,4 @@ One decision per file, append-only; supersede rather than rewrite.
 | [0016](0016-imagegen-interface.md) | imagegen — a canonical text-to-image interface | Accepted |
 | [0017](0017-audio-interfaces.md) | audio — canonical speech synthesis + transcription interfaces | Accepted |
 | [0018](0018-imagegen-editor.md) | imagegen.Editor — image-to-image as a separate optional interface | Accepted |
+| [0019](0019-videogen-interface.md) | videogen — canonical video-generation surface | Accepted |