Add comprehensive OSS-Fuzz integration with 19 fuzzers

[skypher]

Summary

This PR adds a complete fuzzing infrastructure for integration with Google's OSS-Fuzz continuous fuzzing service, targeting all major MuPDF attack surfaces.

Fuzzers Added (19 total)

Category	Fuzzers
Document formats	fuzz_image, fuzz_svg, fuzz_xps, fuzz_epub, fuzz_cbz, fuzz_html
Parsers	fuzz_xml, fuzz_html5, fuzz_json
PDF subsystems	fuzz_pdf_lexer, fuzz_pdf_object, fuzz_pdf_stream, fuzz_cmap, fuzz_stext
Resources	fuzz_font, fuzz_colorspace, fuzz_archive, fuzz_filter, fuzz_path

Also Includes

• 11 dictionaries: Format-specific tokens for mutation guidance (PDF, SVG, XML, HTML, JSON, etc.)
• Seed corpus: Minimal valid files for each format to bootstrap fuzzing
• build.sh: OSS-Fuzz build orchestration script
• Makefile: Local fuzzer development and testing
• Multi-engine support: Compatible with libfuzzer, AFL++, and honggfuzz
• All sanitizers: ASan, MSan, and UBSan

Testing

• All 19 fuzzers compile successfully against MuPDF
• Tested with OSS-Fuzz infrastructure locally (check_build passes for all 20 fuzzers)
• fuzz_image includes OOM protection (MAX_IMAGE_DIM check)
• fuzz_path exercises both path construction and rasterization

Files

fuzz/
├── fuzz_*.c          # 19 fuzzer source files (with AGPL headers)
├── build.sh          # OSS-Fuzz build script
├── Makefile          # Local development makefile
├── dictionaries/     # 11 format-specific dictionaries
└── corpus/           # Seed files for each fuzzer

Next Steps

After this merges, a corresponding PR will be submitted to google/oss-fuzz with the build integration (Dockerfile, project.yaml).