GIF LZW uses "early change" — the code size must bump when next_code
reaches (1 << code_size), not after it exceeds it. Without this fix,
the decoder would read codes with the wrong bit width for GIFs with
enough unique codes to trigger a code size increase.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Pure Rust GIF decoder in the image crate supporting:
- GIF87a and GIF89a format parsing
- LZW decompression with variable-width codes, clear/EOI codes
- Global and local color tables
- Graphic Control Extension (transparency, delay, disposal methods)
- Multi-frame animated GIFs with canvas compositing
- Interlaced image support (4-pass Adam7-style deinterlacing)
- Sub-frame positioning (frames smaller than logical screen)
- Application, comment, and plain text extension skipping
- API: decode_gif() for first frame, decode_gif_frames() for all frames

31 unit tests covering LZW decompression, deinterlacing, frame decoding,
transparency, animation, local color tables, extensions, and error cases.

No external dependencies, no unsafe.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Pure Rust PNG decoder supporting all color types (grayscale, RGB,
indexed, grayscale+alpha, RGBA), bit depths 1-16, scanline filtering
(None/Sub/Up/Average/Paeth), Adam7 interlacing, tRNS transparency,
CRC-32 validation, and multiple IDAT chunk concatenation.

40+ unit tests covering all color types, bit depths, filter types,
interlacing, transparency, error cases, and edge cases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>