Limit individual allocator requests made by PNG decoders to the Kitty
graphics protocol's 400 MiB image ceiling. Add a reusable allocator
wrapper for callers that need per-request bounds.
PNG decoding previously used Wuffs' 4 GiB package limit and checked
the result only after allocation. A tiny PNG with oversized dimensions
could cause a multi-gigabyte RSS spike before being rejected.
Wrap decoder allocators with LimitedAllocator and translate limit
rejections to invalid image data while preserving genuine out-of-memory
errors. Add allocator boundary tests and regression coverage for a
crafted PNG below Wuffs' limit.
