fix: prevent integer overflow and negative distance in position calculation

[nh13]

Summary

• Clamp distance d to valid range to prevent negative/invalid values from normal distribution
• Use 64-bit arithmetic for range calculation to prevent integer overflow
• Skip iteration if calculated range is invalid

Closes #96

Root Cause

When generating paired-end reads, the distance d is calculated from a normal distribution. With extreme values (e.g., -10 sigma), d can become negative, causing:

1. The expression (l - d + 1) to overflow
2. Reads to be generated from incorrect positions

Changes

Both occurrences in dwgsim_core() are fixed:

• Lines 646-660 (standard case)
• Lines 669-683 (regions_bed case)

Test plan

• Verify build succeeds
• Run integration tests (make test-integration)
• Manually test with high std_dev values to verify no crashes

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 29c3b7d and bee7150.

📒 Files selected for processing (1)

• src/dwgsim.c

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/integer-overflow-position-calculation

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[nh13]

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