Implement the DCTDecode (JPEG) passthrough filter with marker validation
and /ColorTransform metadata parsing.
Changes:
- Add StreamInvalidJpeg diagnostic code for missing SOI/EOI markers
- Implement DCTDecoder struct with:
- SOI (0xFFD8) marker validation
- EOI (0xFFD9) marker validation
- /ColorTransform parameter parsing
- Raw byte passthrough with bomb limit enforcement
- Replace PassthroughDecoder with DCTDecoder in get_decoder()
- Add comprehensive test coverage (6 test cases)
The decoder validates JPEG markers but passes through data even when
markers are missing (INV-8 error recovery). Diagnostics are emitted
for missing markers but currently dropped due to trait limitations
(future enhancement will add diagnostics buffer to StreamDecoder).
Closes: pdftract-66dd8
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
