fix(consensus): collapse codec masking if into a let-chain

[nh13]

Summary

Main's lint job is currently failing — commit 0ae197c0 (#575, the MSRV/let-chain adoption) bumped the toolchain to Rust 1.93, whose clippy now fires collapsible_if on a pre-existing nested if in mask_consensus_quals_query_based (crates/fgumi-consensus/src/codec_caller.rs:1315). #575 didn't clean it up, so main and every PR rebased onto it fail cargo ci-lint.

This collapses the nested if outer_bases_length > 0 { if let Some(outer_qual) = outer_bases_qual } into a single let-chain — the idiom the newer toolchain unlocks — matching the project policy in CLAUDE.md to adopt new idioms rather than suppress the clippy lints that flag them. Behavior is unchanged.

Verification

• cargo ci-lint (workspace, -D warnings -W clippy::pedantic) passes clean.
• No functional change; the masking logic is byte-for-byte equivalent.

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 0ae197c and 5d428fe.

📒 Files selected for processing (1)

• crates/fgumi-consensus/src/codec_caller.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nh/fix-codec-collapsible-if

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.96%. Comparing base (0ae197c) to head (5d428fe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #590      +/-   ##
==========================================
- Coverage   92.99%   92.96%   -0.04%     
==========================================
  Files         167      167              
  Lines      103267   103266       -1     
==========================================
- Hits        96034    96000      -34     
- Misses       7233     7266      +33     

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