feat(pdftract-1bv81): implement ASCII85Decode filter per PDF spec 7.4.3
- Add DiagCode::StructInvalidAscii85 diagnostic code
- Fix ASCII85Decode to use PDF spec 7.2.2 whitespace (not Rust's is_ascii_whitespace)
- Add overflow checking on accumulator computation
- Fix 'z' shortcut handling (only valid at count == 0, skip mid-group)
- Fix invalid byte handling (skip and continue per INV-8)
- Add comprehensive test coverage: z shortcut, odd final groups, PDF whitespace,
invalid bytes, bomb limit, empty stream, no delimiters, full range, roundtrip
Acceptance criteria:
- Round-trip: encode 1 KB random bytes via reference ASCII85 encoder, decode → byte-identical ✓
- z shortcut: decoding "zz" produces 8 zero bytes ✓
- Odd final group: <~5sdp~> decodes to "ABC" ✓
- Bytes outside valid range are skipped, decoder continues ✓
- PDF whitespace (NUL, HT, LF, FF, CR, Space) ignored ✓
- <~s8W-!~> decodes to [0xFF, 0xFF, 0xFF, 0xFF] ✓
Closes: pdftract-1bv81
d9d60b1de2