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/lipsync.go

@@ -57,16 +57,16 @@ func (r LipsyncRequest) Apply(opts ...LipsyncOption) LipsyncRequest {
 	return r
 }
 
-// LipSyncer animates still portraits into talking-head clips. Its own small
+// Lipsyncer animates still portraits into talking-head clips. Its own small
 // interface rather than a method on Model: lip-syncers are not text-to-video
 // generators — they bind to a different backend id entirely.
-type LipSyncer interface {
+type Lipsyncer interface {
 	// Lipsync returns the talking-head clip. Generation is slow (minutes);
 	// bound the call with a context deadline.
 	Lipsync(ctx context.Context, req LipsyncRequest, opts ...LipsyncOption) (*Result, error)
 }
 
-// LipsyncModelOption configures a LipSyncer at construction time. Reserved
+// LipsyncModelOption configures a Lipsyncer at construction time. Reserved
 // for future per-model settings.
 type LipsyncModelOption func(*LipsyncModelConfig)
 
@@ -82,12 +82,12 @@ func ApplyLipsyncModelOptions(opts []LipsyncModelOption) LipsyncModelConfig {
 	return cfg
 }
 
-// LipsyncProvider mints LipSyncers bound to one backend.
+// LipsyncProvider mints Lipsyncers bound to one backend.
 type LipsyncProvider interface {
 	// Name is the registry identifier for the provider.
 	Name() string
 
-	// LipsyncModel returns a LipSyncer bound to the given id (passed through
+	// LipsyncModel returns a Lipsyncer bound to the given id (passed through
 	// to the backend verbatim; no catalog validation).
-	LipsyncModel(id string, opts ...LipsyncModelOption) (LipSyncer, error)
+	LipsyncModel(id string, opts ...LipsyncModelOption) (Lipsyncer, error)
 }