refactor(umi): remove redundant allocations, use byte comparison

[nh13]

Summary

• Replace TagSets::consensus_reverse() and consensus_revcomp() methods (which allocated Vec<String> on every call) with CONSENSUS_REVERSE and CONSENSUS_REVCOMP static slice constants
• Simplify IdentityUmiAssigner::assign to uppercase each UMI once instead of twice
• Change count_mismatches from .chars().zip() to .as_bytes().iter().zip() for more efficient byte-level comparison on ASCII UMI sequences

Test plan

• cargo nextest run -p fgumi-umi — all tests pass
• cargo clippy -p fgumi-umi --all-features -- -D warnings — no warnings
• cargo check (full workspace) — clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.50%. Comparing base (fa96485) to head (d52f3f2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   83.50%   83.50%   -0.01%     
==========================================
  Files         126      126              
  Lines       51375    51375              
==========================================
- Hits        42901    42899       -2     
- Misses       8474     8476       +2     

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR refactors the UMI assignment logic to compare bytes instead of chars and improves canonical UMI deduplication by using HashSet for explicit deduplication. Two public helper functions are replaced with public constants, reducing allocations and simplifying the API. Tests and internal usage are updated accordingly to match the new constant-based approach.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly outlines all three key changes (constant vectors, simplified assign logic, and byte comparison) with test validation confirming implementation.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title accurately summarizes the main changes: removing redundant allocations and switching to byte comparison. It covers the primary refactoring goals.

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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 refactor/nh/simplify-fgumi-umi

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