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 —
-a nil `InitImage` is a pure text prompt; setting it conditions generation
-on that frame (hybrid checkpoints like Wan 2.2 TI2V serve both). First
-backend: llama-swap (blocking `/v1/videos/sync`, vLLM-Omni style — the
+defaults, bytes in/out. All modes are one surface, selected by which
+keyframes are set rather than by a mode flag:
+
+| `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"},