v1.2 — ISO/CSO, multi-file trees, method variants, scanner corpus

v1.2 changes

• New formats: ISO 9660 (single-file + multi-file tree) and CSO (decompress → ISO byte stream)
• Multi-file tree archives: normal/rawfile_tree/ (iso + tar + gz/bz2/xz/zst), asserted via expected_files + tree_sha256
• Method/feature variants: 7z LZMA/PPMd/BZip2/Copy/BCJ, zip stored/BZip2/LZMA + AES-256, multi-member gzip/zstd, zstd no-check/no-FCS, xz sha256/block/delta/x86 filters, lz4 content-size/legacy, lzma known-size header, rar store/solid, split zip byte-volumes
• Scanner corpus (scan/): embedded-at-offset gz/xz/lzma/zip + 7 scanner rawfiles (png/gif/tiff/pdf/elf/wav/mpeg) + scan/manifest.json
• faults: 23 negative cases (added truncated zstd/lz4/bz2/brotli/lzma, corrupt zip/rar/xz/lzma, missing 7z volume)
• manifest v1.2: expected_files/tree_sha256 for tree entries; fixed split 7z/zip volumes and split rar levels
• tooling: gen_corpus (Rust), gen_v12.sh, gen_scan.py; verify.sh + Rust/Python/Go harnesses extended

Data files (normal/password/split/rawfiles tarballs) are attached to this release.