A tiny header naming ~1000 channels on a 64M-pixel line sized each chunk
buffer at hundreds of GB and Vec::with_capacity aborted the process.
Chunks over 256 MB are refused, and the RLE / zlib buffers reserve with
try_reserve_exact. Regression test: huge_scan_lines_are_refused_before_allocating.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>