Update capnproto dependency

[harrishancock]

Putting this up mostly just to run CI with my capnproto branch for now.

[codspeed-hq]

Merging this PR will not alter performance

✅ 70 untouched benchmarks
⏩ 129 skipped benchmarks¹

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_{Comparing harris/2026-02-19-re-reland-niche-optimized-maybe (d681bd9) with main (522277a)}

Footnotes

1. 129 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. ↩