fix: guard against missing MC tag in allele frequency calculation

[nh13]

Summary

• Add rec.mateEnd.isDefined check to checkAsPair in AggregateSvPileup.numOverlappingTemplates
• Prevents NoSuchElementException when BAM files lack the MC (mate CIGAR) tag, which is needed to derive mateEnd
• Records without the MC tag now fall through to single-read overlap checking instead of crashing

Test plan

• Existing tests pass (./mill tools.test)
• Verify with a BAM file that lacks MC tags (e.g. older BAMs or certain aligner outputs)

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📥 Commits

Reviewing files that changed from the base of the PR and between ca7b619 and 8187d90.

📒 Files selected for processing (2)

• docs/tools/index.md
• src/main/scala/com/fulcrumgenomics/sv/tools/AggregateSvPileup.scala

✨ Finishing Touches

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• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/mate-end-unsafe-get

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