Move Pair containers to sequence storage

[ezrosent]

Summary

• Encode pairs as [rebuild mask, first, second].
• Construct and project pairs directly through the sequence representation.
• Rebuild only the lanes selected by the pair mask.
• Cover round trips, selective and unchanged rebuilds, and execution-local predictions.

Stack

Draft, stacked on #982. This is PR 10 of 12 in the sequence-backed container stack.

Validation

Validated on the complete 12-commit stack:

• cargo nextest run --workspace: 1,362 tests passed
• cargo test --workspace --doc
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.78744% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.63%. Comparing base (08d400c) to head (959c15a).

Files with missing lines	Patch %	Lines
src/sort/pair.rs	91.78%	17 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           codex/inline-containers-09-multiset     #983      +/-   ##
=======================================================================
- Coverage                                87.77%   87.63%   -0.14%     
=======================================================================
  Files                                       98       98              
  Lines                                    39312    39516     +204     
=======================================================================
+ Hits                                     34506    34631     +125     
- Misses                                    4806     4885      +79     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks
⏩ 248 skipped benchmarks¹

---

_{Comparing codex/inline-containers-10-pair (959c15a) with codex/inline-containers-09-multiset (8422313)²}

Footnotes

1. 248 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on codex/inline-containers-09-multiset (08d400c) during the generation of this report, so a275158 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩