Add side_by_side() for horizontal animation concatenation

[Copilot]

Enables rendering multiple animations horizontally concatenated, playing simultaneously frame-by-frame.

Changes

• side_by_side(streams, uri) - New function in core.py for combining streams horizontally
• SideBySideStreams - New class in streams.py handling frame concatenation logic
  • Uses np.hstack() for efficient horizontal combination
  • Pads shorter streams by repeating last frame to match longest
  • Pads shorter heights with black pixels to align tops
  • Handles GIF (duration arrays) and MP4 (init_video_stream) format differences

Usage

from streamjoy import side_by_side, stream

stream1 = stream(resources=df, x='date', y='temperature', extension=".gif")
stream2 = stream(resources=df, x='date', y='humidity', extension=".gif")

side_by_side([stream1, stream2], uri="comparison.gif")

Works with any number of streams. Returns SideBySideStreams object when uri=None, otherwise writes and returns the path/BytesIO.

Implementation Notes

• Type annotation fixed: returns SideBySideStreams | Path | BytesIO
• __len__() returns max frame count (differs from ConnectedStreams which sums)
• __iter__() returns Iterable[MediaStream] not Iterable[Path]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Side by side animations</issue_title>
<issue_description>Render two animations side by side.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Side by side animations #5

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