fix: aggregate startxref-xref keyword offset tolerance

[vitormattos]

Summary

• Mirror of upstream parser robustness fix for malformed startxref offsets landing inside xref.
• Keeps fork aggregate branch aligned while upstream review runs.

Upstream PR

• fix: tolerate startxref offset inside xref keyword smalot/pdfparser#798

Scope

• Branch: fix/startxref-whitespace-xref-stream
• Target aggregate branch: fork/libresign-parser-fixes\n\nPDF Source
• Corpus: veraPDF-corpus
• Fixture origin: PDF_A-1b/6.1 File structure/6.1.2 File header/veraPDF test suite 6-1-2-t01-fail-a.pdf
• Source URL: https://github.com/veraPDF/veraPDF-corpus/blob/staging/PDF_A-1b/6.1%20File%20structure/6.1.2%20File%20header/veraPDF%20test%20suite%206-1-2-t01-fail-a.pdf
• Local copy used in tests: samples/bugs/PullRequestStartxrefWhitespaceXrefStream.pdf