🐛 fix(xslt): resolve file URL imports

[gaborbernat]

xsl:import failed when base_url was a local file:// stylesheet URL. The resolver looked for file:/.../base.xsl.

Convert local file URLs before resolving imports, keep remote URLs rejected, and update the docs.

[codspeed-hq]

Merging this PR will not alter performance

✅ 91 untouched benchmarks
⏩ 18 skipped benchmarks¹

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_{Comparing gaborbernat:fix/xslt-base-url-contract (07f4a74) with main (da688c5)}

Footnotes

1. 18 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩