fix(avif): check alpha and primary picture dimensions match

[MsfPablo]

Summary

Fixes #3084. The primary and alpha pictures in an AVIF file are decoded by separate dav1d instances, so nothing guarantees they agree on size. read_image zipped the primary buffer against the alpha plane without checking this — a malformed file with mismatched primary/alpha dimensions would silently truncate compositing to the shorter of the two, instead of erroring on the malformed input.

The 16-bit path (process_16bit_picture) had the same underlying gap in a different shape: it reshapes the alpha plane via transmute_y_plane16 using the primary picture's width/height, without ever confirming the alpha picture's actual dimensions match those.

Fix

Added a dimension check alongside the existing pixel_layout check, in both the 8-bit and 16-bit alpha-compositing paths, returning a new AvifDecoderError::AlphaPlaneSize { alpha, primary } variant on mismatch instead of silently truncating.

Test plan

• cargo build --features avif -p image and cargo test --features avif -p image — clean, existing truncate_avif test still passes, no regression
• cargo fmt -p image -- --check and cargo clippy --features avif -p image -- -D warnings — both clean

I was not able to add a dedicated regression test for the mismatch case itself: reproducing it requires a malformed AVIF container with two AV1 sub-images (primary + alpha) at different resolutions, and I didn't have a reliable way to construct or find such a fixture. Rather than fabricate something that doesn't actually exercise the mismatch path, I'm disclosing that gap here — happy to add one if pointed at an existing fixture-generation pattern for this codec, or if a maintainer can supply/generate a suitable malformed sample.

Disclosure

Per this org's CONTRIBUTING.md LLM policy: this PR was substantially developed with AI assistance (Claude), under my direction and review — I investigated the reported root cause myself, wrote/reviewed the fix, and ran the verification above locally before opening this.

[197g]

Thank you, LGTM (no regression test is okay to me here).