fix: aggregate multi-space xref subsection support

[vitormattos]

Summary

• Mirror of upstream fix for xref subsection parsing with multi-space separators.
• Keeps fork aggregate branch aligned while upstream review runs.

Upstream PR

• fix: accept multi-space xref subsection entries smalot/pdfparser#799

Scope

• Branch: fix/xref-subsection-multi-space
• Target aggregate branch: fork/libresign-parser-fixes\n\nPDF Source
• Corpus: veraPDF-corpus
• Fixture origin: Isartor test files/PDFA-1b/6.1 File structure/6.1.4 Cross reference trailer/isartor-6-1-4-t01-fail-a.pdf
• Source URL: https://github.com/veraPDF/veraPDF-corpus/blob/staging/Isartor%20test%20files/PDFA-1b/6.1%20File%20structure/6.1.4%20Cross%20reference%20trailer/isartor-6-1-4-t01-fail-a.pdf
• Local copy used in tests: samples/bugs/PullRequestXrefSubsectionMultipleSpaces.pdf