Add variable-arity sequence tables

[ezrosent]

Summary

• Add SequenceTable, a SortedWritesTable analogue with variable-length keys and a fixed-width non-key value tail.
• Preserve mutation-buffer, Subset, version, merge, and generation semantics using packed values plus cumulative row offsets.
• Support serial and parallel insertion, deletion, rebuilding, deduplication, and compaction, including empty and mixed-arity rows.
• Cover layout invariants, duplicate handling, cloning, staleness, hash collisions, rebuilds, and parallel compaction.

Stack

Draft, stacked on #966. This is PR 1 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 93.69724% with 112 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.56%. Comparing base (f93fb49) to head (1b22eca).

Files with missing lines	Patch %	Lines
core-relations/src/table/sequence.rs	93.68%	112 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           codex/vectorized-free-join-on-962     #974      +/-   ##
=====================================================================
+ Coverage                              87.27%   87.56%   +0.28%     
=====================================================================
  Files                                     96       97       +1     
  Lines                                  34407    36183    +1776     
=====================================================================
+ Hits                                   30030    31683    +1653     
- Misses                                  4377     4500     +123     

☔ 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 improve performance by 5.12%

⚠️ 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-2mm]	8.3 s	7.9 s	+5.12%

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-01-sequence-table (1b22eca) with codex/vectorized-free-join-on-962 (f93fb49)}

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. ↩