Move Set containers to sequence storage

[ezrosent]

Summary

• Encode sets as a rebuild flag followed by sorted, unique values.
• Implement reads and edits directly over sequence slices.
• Implement union, difference, and intersection as linear merges of canonical sorted sequences.
• Cover the slice algebra exhaustively against BTreeSet and add nested dirty-propagation tests.

Stack

Draft, stacked on #980. This is PR 8 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 92.87599% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.87%. Comparing base (4a80623) to head (0481cf8).

Files with missing lines	Patch %	Lines
src/sort/set.rs	92.87%	27 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           codex/inline-containers-07-map     #981      +/-   ##
==================================================================
+ Coverage                           87.84%   87.87%   +0.02%     
==================================================================
  Files                                  98       98              
  Lines                               38368    38738     +370     
==================================================================
+ Hits                                33706    34040     +334     
- Misses                               4662     4698      +36     

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[codspeed-hq]

Merging this PR will improve performance by ×5.8

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tests[repro-665-set-union]	634.8 ms	109.2 ms	×5.8

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-08-set (0481cf8) with codex/inline-containers-07-map (0c46b35)²}

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. ↩

2. No successful run was found on codex/inline-containers-07-map (4a80623) during the generation of this report, so ba5c36e was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩