docs(videogen): gadfly — README FL2V section, and stop pointing at a note that does not exist

- README documented only t2v/i2v. Now a table of the four keyframe
  combinations, plus the undetectable-support caveat, which is the one thing a
  caller cannot work out for itself.
- The LastImage doc comment said "see the note on LastImage support in
  provider/llamaswap" — there was no such note. A pointer to something that
  does not exist is worse than no pointer; the comment is now self-contained.
- Generate's doc described only input_reference; it now names
  input_reference_last and explains why an unsupporting backend returns a clip
  rather than an error.

The 2/4 finding (writeImagePart reusing the "frame" base for both parts) was
already fixed in dbc9689 — from the receiving end, where the consequence is
concrete rather than stylistic: ComfyUI stages uploads by FILENAME with
overwrite=true, so a shared name means the second clobbers the first and both
keyframes resolve to one image.

Not taken: initImageFilename's name is no longer misleading (writeImagePart
stopped calling it), and it is still used by lipsync.go so it is not dead.
The empty-LastImage test stays standalone — it mirrors the existing standalone
empty-InitImage coverage rather than a table this file does not have.

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

provider/llamaswap/video.go

@@ -38,10 +38,13 @@ type videoModel struct {
 // bound the call with a context deadline.
 //
 // Parameter names follow vLLM-Omni's videos API (num_frames, fps,
-// num_inference_steps, guidance_scale); the conditioning frame is sent as an
-// `input_reference` file part, following OpenAI's videos API. Upstreams
+// num_inference_steps, guidance_scale); the leading conditioning frame is sent
+// as an `input_reference` file part, following OpenAI's videos API, and a
+// trailing keyframe (Request.LastImage) as `input_reference_last`. Upstreams
 // ignore fields they don't understand, and optional fields stay off the wire
-// entirely so the model's own defaults apply.
+// entirely so the model's own defaults apply — which is also why a backend
+// without first-last-frame support returns an ordinary clip here rather than
+// an error.
 func (m *videoModel) Generate(ctx context.Context, req videogen.Request, opts ...videogen.Option) (*videogen.Result, error) {
 	req = req.Apply(opts...)
 	if strings.TrimSpace(req.Prompt) == "" {