Add property-based testing infrastructure for the lexer module with 6+
property tests covering INV-8 (no panic), string/hex roundtrips, name
length bounds, and position monotonicity. Create 8 curated fixture files
with golden token outputs for critical edge cases including EC-01 empty
file test and whitespace-only inputs.
Changes:
- Add prop_string_roundtrip to tests/proptest/lexer.rs
- Create tests/lexer/fixtures/ with 8 fixtures + .tokens.txt golden files
- Add gen_lexer_golden.rs binary for regenerating golden outputs
- Fix missing ObjRef import in marked_content_operators.rs
Acceptance criteria:
- cargo test --features proptest -p pdftract-core: 105 lexer tests pass
- tests/lexer/fixtures/ contains 8 fixtures with .tokens.txt outputs
- EC-01 empty file test: 0-byte input -> Token::Eof, no panic
- Whitespace-only file test passes
- INV-8 verified by prop_lexer_never_panics
Closes: pdftract-sy8x
585d861efc