fix: accept multi-space xref subsection entries

[vitormattos]

Summary

• Fixes parsing of xref tables where subsection/entry fields use more than one space separator.
• Prevents losing xref object map and avoids downstream Object list not found on valid but non-canonical files.
• Adds integration fixture and regression test.

Reproduction

• Fixture parsed by pdfinfo but parser produced Object list not found. Possible secured file..
• Root cause: strict regex expected single-space separators in xref lines.

Changes

• Relax xref line matching in decodeXref() to accept one or more spaces.
• Add fixture: samples/bugs/PullRequestXrefSubsectionMultipleSpaces.pdf.
• Add test: testParseFileWithXrefSubsectionHavingMultipleSpaces.\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

[vitormattos]

This standalone PR has been restacked into the RawDataParser consolidation chain to keep a single merge path per source-file group.\n\nSuperseded-by chain:\n- upstream base: #796\n- fork consolidation into base: https://github.com/vitormattos/pdfparser/pull/28\n- stack continuation: https://github.com/vitormattos/pdfparser/pull/29\n\nClosing this standalone PR to avoid duplicate merge routes.