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

README.md

@@ -272,17 +272,30 @@ tr, err := tm.Transcribe(ctx, audio.TranscriptionRequest{
 voices, err := ls.ListVoices(ctx, "kokoro") // []string of voice ids
 ```
 
-## Video: text-to-video + image-to-video
+## Video: text-to-video, image-to-video, first-last-frame
 
 Video generation lives in the `videogen` package (ADR-0019), mirroring
 imagegen/audio: one small `Model` contract, zero values mean backend
-defaults, bytes in/out. Text-to-video and image-to-video are one surface —
+defaults, bytes in/out. All modes are one surface, selected by which
-a nil `InitImage` is a pure text prompt; setting it conditions generation
+keyframes are set rather than by a mode flag:
-on that frame (hybrid checkpoints like Wan 2.2 TI2V serve both). First
+
-backend: llama-swap (blocking `/v1/videos/sync`, vLLM-Omni style — the
+| `InitImage` | `LastImage` | mode |
+|---|---|---|
+| nil | nil | text-to-video |
+| set | nil | image-to-video (hybrid checkpoints like Wan 2.2 TI2V serve both) |
+| set | set | first-last-frame — both ends pinned |
+| nil | set | pin the destination, model invents the approach |
+
+First backend: llama-swap (blocking `/v1/videos/sync`, vLLM-Omni style — the
 response body is the encoded clip, so `Result` carries a single `Video`).
 Generation runs for minutes; bound the call with a context deadline.
 
+**`LastImage` support is per-model and cannot be detected.** A backend that
+does not understand a trailing keyframe ignores the part and returns an
+ordinary clip — indistinguishable from success. There is no capability bit,
+because the contract has no way to learn one, so a caller depending on the
+pin must establish support out of band.
+
 ```go
 vm, _ := ls.VideoModel("videogen-wan22-5b")
 res, err := vm.Generate(ctx, videogen.Request{Prompt: "a cat surfing"},

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
+// num_inference_steps, guidance_scale); the leading conditioning frame is sent
-// `input_reference` file part, following OpenAI's videos API. Upstreams
+// 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) == "" {

videogen/videogen.go

@@ -59,10 +59,11 @@ type Request struct {
 	//
 	// Support is per-model and NOT advertised anywhere in this contract: a
 	// backend that does not understand a trailing keyframe ignores it and
-	// returns an ordinary clip, which is indistinguishable from success. A
+	// returns an ordinary clip, which is indistinguishable from success.
-	// caller that needs to know whether the pin took effect must establish
+	// There is no capability bit to consult, because the contract has no way
-	// that out of band — see the note on LastImage support in
+	// to learn one. A caller that needs to know whether the pin actually took
-	// provider/llamaswap.
+	// effect must establish that out of band — by configuration it controls,
+	// not by inspecting the result.
 	LastImage *Image
 
 	// Size is the requested resolution, e.g. "1280x704"; "" = backend default.