fix: review findings — chain NaN/Inf + id hygiene, percent-escape jobPath, shared singleVideoResult

- SubmitChain rejects NaN/±Inf segment seconds with ErrUnsupported
  (previously an obscure json.Marshal error; NaN fails every comparison
  and +Inf passed the >= 0 check).
- ChainStatus skips segment entries with no usable id — JSON null
  (which no-op-unmarshals into a string, previously appending ""),
  empty strings, and id-less objects; the unfiltered list survives in
  Raw. ChainJob.SegmentIDs doc now also says ChainSegmentResult takes
  the segment index, not an id string.
- jobPath rejects '%' in job ids — %2F/%2E%2E percent-escapes decode
  back into path structure server-side, bypassing the literal check on
  this upstream-echoed value.
- singleVideoResult moves to video.go next to videoMIME, and the two
  remaining hand-rolled copies of the video-result validation
  (videoModel.Generate, Interpolate) now use it — one validation, one
  message shape.
- videogen.LipSyncer renamed to videogen.Lipsyncer for consistency with
  the rest of the surface's Lipsync* naming (LipsyncProvider,
  LipsyncModel, LipsyncRequest); not yet consumed downstream, so the
  rename is free now and never again.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WWCQcYStWXBYUy5sZWnbLT

provider/llamaswap/mediautil.go

@@ -207,12 +207,5 @@ func (m *interpolatorModel) Interpolate(ctx context.Context, req videogen.Interp
 	if err != nil {
 		return nil, err
 	}
-	if len(raw) == 0 {
-		return nil, &llm.APIError{Provider: m.p.name, Model: m.id, Message: "interpolate response contained no video"}
-	}
-	mimeType := videoMIME(respType, raw)
-	if mimeType == "" {
-		return nil, &llm.APIError{Provider: m.p.name, Model: m.id, Message: "interpolate response is not a video"}
-	}
-	return &videogen.Result{Video: videogen.Video{Data: raw, MIME: mimeType}}, nil
+	return singleVideoResult(m.p.name, m.id, "interpolate", raw, respType)
 }