[WyattBlue]

More stub fixes, found by type-checking examples/, comparing each class's runtime dunders against its stub, and comparing stream attributes with the codec-context attributes they forward to.

Things that work at runtime but failed type-checking

• SubtitleSet items (rects, __iter__, __getitem__) were typed as the base Subtitle, but at runtime they are only ever AssSubtitle or BitmapSubtitle, so sub.ass could never type-check (examples/subtitles/remux.py failed). Now AssSubtitle | BitmapSubtitle, which also lets a sub.type check narrow the type.
• BitmapSubtitle has __len__, __iter__, and __getitem__ over its planes at runtime; they're now in the stub.
• BitmapSubtitlePlane supports the buffer protocol; added __buffer__, so memoryview(plane) and bytes(plane) type-check.
• VideoFormat has __int__ (used by Graph.add_buffer); added to the stub.

Annotation fixes

• CodecContext.encode, encode_lazy, and decode had no return types. decode returns Sequence[Frame | AssSubtitle | BitmapSubtitle]; I used Sequence because list is invariant, and the list[VideoFrame] that VideoCodecContext.decode returns wouldn't fit a list[Frame] base.
• SubtitleCodecContext.decode(None) raises RuntimeError: packet cannot be None, so its stub now requires a Packet. This uses # type: ignore[override], like the existing encode overrides in the audio and video contexts. SubtitleStream.decode(None) still accepts None, since it substitutes an empty packet.
• VideoStream.format is now VideoFormat | None, matching the VideoCodecContext.format it forwards to.
• Audio, video, and subtitle streams declare name: str, since they forward it to the codec context.
• VideoFrame.to_image returns PIL.Image.Image, and set_image/from_image take one. Pillow is in the test and lint dependency groups.

Notes

• Possible break for type-checked code: stream.format.name on a VideoStream now needs a None check. Four test blocks gained assert stream.format is not None.
• examples/subtitles/remux.py now checks isinstance(sub, AssSubtitle) before using sub.ass.
• Not changed: AudioStream.format/AudioCodecContext.format can also be None at runtime but are typed as always AudioFormat.