fix(sam): use safe conversions for CIGAR ref span and revcomp string

[nh13]

Summary

• Use usize::try_from() instead of as usize cast for CIGAR-derived reference span in regenerate_alignment_tags_raw to avoid silent wrapping on negative values
• Replace String::from_utf8().expect() with from_utf8_lossy() in revcomp_buf_value to match reverse_buf_value pattern and prevent potential panics
• Remove stale # Panics doc section from revcomp_buf_value

Follow-up to #147 addressing CodeRabbitAI review feedback.

Test plan

• All 1852 tests pass (cargo ci-test)
• Formatting clean (cargo ci-fmt)
• Linting clean (cargo ci-lint)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between bf4b24f and 455205d.

📒 Files selected for processing (2)

• crates/fgumi-sam/src/alignment_tags.rs
• crates/fgumi-sam/src/lib.rs

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📝 Walkthrough

Walkthrough

Two functions received defensive improvements. regenerate_alignment_tags_raw now uses usize::try_from() with explicit error handling instead of casting when computing reference span from CIGAR data, rejecting negative values. revcomp_buf_value switched from from_utf8 with expect() to from_utf8_lossy(), removing a panic path and returning valid UTF-8 regardless. Documentation was updated to reflect these changes. No public APIs were modified.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and accurately reflects the main changes: replacing unsafe casts with safe conversions for CIGAR reference span and revcomp string operations.
Description check	✅ Passed	The description clearly relates to the changeset, detailing the three specific improvements made and referencing the follow-up context.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch 147/nh/fix-sam-safety

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.81%. Comparing base (bf4b24f) to head (455205d).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
- Coverage   83.81%   83.81%   -0.01%     
==========================================
  Files         126      126              
  Lines       51211    51211              
==========================================
- Hits        42924    42923       -1     
- Misses       8287     8288       +1     

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