fix(gif): compose sub-rectangle frames so the animation actually plays

[LeadcodeDev]

Closes #185.

What

A gif component showed its first frame and then nothing. No error, no warning — the scenario validated and the render succeeded.

ffmpeg -f lavfi -i "testsrc2=size=64x64:rate=12:duration=2" anim.gif   # 24 frames
rustmotion render -f gif.json -o f.png --frame 0    # 16140 non-black pixels
rustmotion render -f gif.json -o f.png --frame 5    #     0 non-black pixels

Cause

Every frame after the first is usually a sub-rectangle holding only the pixels that changed. The decoder stored frame.buffer alongside the canvas dimensions, so the buffer was shorter than width * height * 4. raster_from_data returned None, the if let Some(img) arm was skipped, and nothing was drawn or said. The first frame is full-canvas in most encoders — which is why it was the only one that ever appeared.

How

decode_composed_frames composes each frame onto a persistent canvas at (left, top):

• alpha 0 means "leave what is underneath" — GIF transparency is an index, not a channel, and that is the whole point of the sub-rectangle encoding;
• disposal is honoured: Background clears the frame's rect, Previous restores the canvas captured before compositing, Keep/Any leave it;
• the rect is clamped to the canvas, so a malformed GIF cannot panic the blit.

Even at the right size, drawing an optimised frame on its own would have shown a fragment against an empty background — the composition is the fix, not just the sizing.

Two smaller corrections came with it: the three silent returns now report once per source (the treatment #181 gave the audio analysis), and AlphaType is Unpremul because the decoder emits straight RGBA.

After

frame  0 -> 16140 non-black   fingerprint 2db960
frame  5 -> 16128 non-black   fingerprint a26a0c
frame 10 -> 16104 non-black   fingerprint ae6838
frame 20 -> 16056 non-black   fingerprint ab2845

Tests

gif had none. Three added, on a fixture written with the gif encoder — frame 0 full canvas, frame 1 a 2x2 sub-rectangle with Keep:

• both frames are drawable, full-canvas, and differ; the kept half stays red and the patched half turns blue
• a missing file returns None and claims its warn-once slot — silence was the bug, so the test asserts the reporting, not just the return
• a rect running past the canvas is clamped

cargo test --workspace green, cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.