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

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
-	// caller that needs to know whether the pin took effect must establish
-	// that out of band — see the note on LastImage support in
-	// provider/llamaswap.
+	// returns an ordinary clip, which is indistinguishable from success.
+	// There is no capability bit to consult, because the contract has no way
+	// to learn one. A caller that needs to know whether the pin actually took
+	// 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.