fix: tolerate startxref offset near xref keyword

[vitormattos]

Summary

Tolerate startxref offsets that land on whitespace immediately before the xref keyword, or one byte into xref.

Why

Some malformed PDFs do not point startxref exactly at the beginning of xref, even though the cross-reference table is still recoverable.

Without this tolerance, parsing can fail with Invalid object reference for $obj. on files that should still be recoverable.

Validation

Validated against representative regressions from the corpora used in the larger PDF.js integration work:

• PDF.js: outlines_for_editor.pdf
• VeraPDF: veraPDF test suite 6-6-2-3-2-t01-pass-c.pdf
• VeraPDF: veraPDF test suite 6-1-2-t01-fail-a.pdf

Notes

This is a focused fix and can be reviewed independently from the aggregate integration PR.

[vitormattos]

Superseded by the consolidation chain in the fork: vitormattos#36\n\nThis includes PR810 parser fix and regression coverage, integrated on top of PR808 stack with fixture/test consolidation.