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

videogen/chain.go

@@ -42,9 +42,13 @@ type ChainJob struct {
 	Segment int
 	Total   int
 
-	// SegmentIDs are the COMPLETED per-segment artifacts, fetchable via
-	// ChainSegmentResult — a mid-chain failure still leaves these
-	// retrievable, so multi-minute GPU output is never discarded.
+	// SegmentIDs name the COMPLETED per-segment artifacts, in order. A
+	// mid-chain failure still leaves those segments retrievable via
+	// ChainSegmentResult — note it takes the segment's index in the chain,
+	// not an id string; this list tells you WHICH segments completed. So
+	// multi-minute GPU output is never discarded. Entries the backend
+	// reports without a usable id are skipped (the unfiltered list survives
+	// in Raw).
 	SegmentIDs []string
 
 	// Raw is the provider-native job payload. May be nil.