Implements decode_cjk_bytes() function wrapping encoding_rs for the four
major CJK byte encodings used in legacy PDFs: Shift-JIS, GB18030, Big5, and
EUC-KR. Used by Phase 2.3 fallback path when fonts use raw byte encodings
instead of proper CMap/ToUnicode mappings.
- Add CjkEncoding enum with ShiftJis, Gb18030, Big5, EucKr variants
- Implement decode_cjk_bytes(enc, bytes) -> (String, bool)
- Use decode_without_bom_handling (PDF byte streams never have BOM)
- Return bool indicating malformed bytes for caller to emit diagnostic
- Add 15 tests covering valid input, malformed input, empty input, round-trips
Supporting changes:
- Add encoding_rs dependency (optional, gated by cjk feature)
- Add CjkDecodeMalformed diagnostic code
- Export CjkEncoding and decode_cjk_bytes from font module
Refs: pdftract-5rmc, plan.md Phase 2.3 (lines 1382-1386)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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