Move Vec containers to sequence storage

[ezrosent]

Summary

• Encode vectors canonically as [rebuild flag, elements...].
• Run vector reads, edits, and append directly over borrowed sequence slices.
• Rebuild only eligible element lanes and use the occurrence index for dirty propagation.
• Add integration coverage for containers observed before and after relation rebuilds.

Stack

Draft, stacked on #978. This is PR 6 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 86.76471% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.69%. Comparing base (ea2856a) to head (d8a18cf).

Files with missing lines	Patch %	Lines
src/sort/vec.rs	86.76%	36 Missing ⚠️

Additional details and impacted files

@@                                Coverage Diff                                 @@
##           codex/inline-containers-05-unified-maintenance     #979      +/-   ##
==================================================================================
+ Coverage                                           87.57%   87.69%   +0.12%     
==================================================================================
  Files                                                  98       98              
  Lines                                               37705    37945     +240     
==================================================================================
+ Hits                                                33019    33275     +256     
+ Misses                                               4686     4670      -16     

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[codspeed-hq]

Merging this PR will improve performance by 17.68%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 36 untouched benchmarks
⏩ 227 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tests[eggcc-extraction]	1.6 s	1.3 s	+17.68%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing codex/inline-containers-06-vec (d8a18cf) with codex/inline-containers-05-unified-maintenance (ea2856a)}

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. ↩