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 87.88%. Comparing base (2b4d183) to head (729c201).

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                                88.01%   87.88%   -0.14%     
=======================================================================
  Files                                      103      103              
  Lines                                    39364    39568     +204     
=======================================================================
+ Hits                                     34645    34773     +128     
- Misses                                    4719     4795      +76     

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[codspeed]

Merging this PR will not alter performance

✅ 37 untouched benchmarks
⏩ 227 skipped benchmarks¹

---

_{Comparing codex/inline-containers-10-pair (729c201) with codex/inline-containers-09-multiset (2b4d183)}

Footnotes

1. 227 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. ↩