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/faceswap.go

@@ -207,10 +207,12 @@ func parseSwapReport(header string) []imagegen.SwappedFace {
 // imageFilename picks a multipart filename for an image part. The shim reads
 // bytes, not names, but a plausible extension keeps server-side sniffing and
 // request logs honest. base distinguishes the parts of a multi-file form
-// ("target"/"source") so a log line says which one was malformed.
+// ("target"/"source", "frame"/"frame_last") so a log line says which one was
+// malformed — and, for the video keyframes, so a backend that stages uploads
+// by filename cannot have the second overwrite the first.
 //
-// initImageFilename (video.go) is this function with base fixed to "frame"
-// and delegates here — two copies of one extension table is how they drift.
+// Every caller routes through here: two copies of one extension table is how
+// they drift.
 func imageFilename(mimeType, base string) string {
 	if base == "" {
 		base = "image"