escapePDFName emitted a single hex digit for character codes below 0x10
(TAB became #9, not #09). PDF 32000-1 7.3.5 requires exactly two hex digits
after #. On re-save (annotations, form fields, font names) such a Name is
written malformed and the lexer mis-parses it on reload, dropping bytes.
Pad the hex to two digits; a no-op for codes 0x10 to 0xFF.
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