feat(pdftract-lhq9t): implement ASCIIHexDecode filter improvements
Implement ASCIIHexDecode filter per PDF spec 7.4.2 with:
- Odd-length final pair handling (pad with low nibble = 0)
- PDF spec whitespace (7.2.2: NUL, HT, LF, FF, CR, Space)
- Invalid byte handling (continue per INV-8)
- Fixed bomb limit enforcement (check BEFORE adding bytes)
Added 11 comprehensive tests covering all acceptance criteria:
- Odd-length: <3> → [0x30], <ABC> → [0xAB, 0xC0]
- Mixed case: <aF> and <Af> both → [0xAF]
- Whitespace ignored: <A B C D> → [0xAB, 0xCD]
- Round-trip: 1 KB random bytes
- Bomb limit enforcement
Closes: pdftract-lhq9t
6aefd76c63