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/video.go

@@ -147,13 +147,6 @@ func singleVideoResult(provider, model, verb string, raw []byte, contentType str
 	return &videogen.Result{Video: videogen.Video{Data: raw, MIME: mimeType}}, nil
 }
 
-// initImageFilename picks the multipart filename hint for the conditioning
-// frame from its MIME subtype. The name is provider-chosen (never
-// caller-supplied), so no sanitization is needed.
-func initImageFilename(mimeType string) string {
-	return imageFilename(mimeType, "frame")
-}
-
 // writeImagePart attaches one conditioning frame under the given field name,
 // with a filename derived from nameStem. Shared by the first- and last-frame
 // parts so the two cannot drift in how they encode, which is the usual way a