refactor(llamaswap): drop initImageFilename — one caller left, and it was a rename of imageFilename

The 2/4 finding is right: after writeImagePart started passing an explicit
filename stem, initImageFilename had no caller in video.go, and its
"conditioning frame" doc no longer described its one remaining user
(lipsync.go's avatar image). A one-line wrapper that survives only to be
misdescribed is not indirection worth keeping.

lipsync now calls imageFilename(mime, "frame") directly, and imageFilename's
doc lists the real bases — including WHY the video keyframes need distinct
ones: a backend that stages uploads by filename would otherwise have the
second overwrite the first.

Not taken: consolidating the first/last-frame rationale to a single canonical
site. The copies address different readers — the wire encoding (provider), the
contract's undetectable-support caveat (videogen), and the mode table (README)
— and last round's finding was a doc pointing at a note that did not exist.
Trading duplication for cross-references is what produced that.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01PLjgrxvHjm1sJgUu9zBPH9

provider/llamaswap/lipsync.go

@@ -56,7 +56,7 @@ func (m *lipsyncModel) Lipsync(ctx context.Context, req videogen.LipsyncRequest,
 	// hand (mirrors videoModel.Generate).
 	var buf bytes.Buffer
 	w := multipart.NewWriter(&buf)
-	fw, err := w.CreateFormFile("image", initImageFilename(req.Image.MIME))
+	fw, err := w.CreateFormFile("image", imageFilename(req.Image.MIME, "frame"))
 	if err != nil {
 		return nil, fmt.Errorf("llama-swap: build lipsync form: %w", err)
 	}