Add two proptests for the PDF hex string lexer to verify robustness
and correctness:
1. proptest_hex_string_never_panics_on_random_bytes: Random byte
sequences starting with '<' (not '<<') never cause panics.
2. proptest_hex_string_roundtrip_via_reencode: Hex decode + re-encode
roundtrip property validates that encoding and decoding are
inverse operations.
The hex string lexer implementation was already present and correct,
with proper handling of odd-length zero padding (<4> -> \x40, not \x04).
All acceptance criteria pass:
- Empty hex string: <> -> b""
- Odd-length single nibble: <4> -> b"\x40" (critical test)
- Standard decoding: <48656C6C6F> -> b"Hello"
- Mixed case: <aBcD> -> b"\xAB\xCD"
- Whitespace ignored: <48 65> -> b"\x48\x65"
- Unterminated with diagnostic: <48 -> b"\x48" + STRUCT_UNTERMINATED_STRING
- Proptests pass: random bytes never panic, roundtrip property holds
- INV-8 maintained: all error paths use diagnostics, no panics
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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