Add sequence-backed container infrastructure

[ezrosent]

Summary

• Represent a container as a SequenceTable row containing [canonical key..., identity].
• Add canonical sequence codecs with direct serialized fast paths and a deserialize/rebuild/serialize compatibility path.
• Add execution-local identity prediction, a sharded reverse map, occurrence-indexed rebuilds, and compatibility with the legacy backend.
• Wire sequence containers through actions, free join, dependency registration, the bridge, and public execution APIs.

Stack

Draft, stacked on #976. This is PR 4 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 85.88640% with 164 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.49%. Comparing base (fd5fa6f) to head (9ee921c).

Files with missing lines	Patch %	Lines
core-relations/src/containers/sequence.rs	80.55%	84 Missing ⚠️
core-relations/src/containers/mod.rs	84.86%	33 Missing ⚠️
egglog-bridge/src/lib.rs	36.00%	16 Missing ⚠️
src/exec_state.rs	23.07%	10 Missing ⚠️
src/lib.rs	0.00%	8 Missing ⚠️
core-relations/src/free_join/mod.rs	96.88%	7 Missing ⚠️
core-relations/src/action/mod.rs	97.52%	5 Missing ⚠️
core-relations/src/dependency_graph.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@                                Coverage Diff                                 @@
##           codex/inline-containers-03-sequence-benchmarks     #977      +/-   ##
==================================================================================
- Coverage                                           87.65%   87.49%   -0.17%     
==================================================================================
  Files                                                  97       98       +1     
  Lines                                               36491    37520    +1029     
==================================================================================
+ Hits                                                31987    32828     +841     
- Misses                                               4504     4692     +188     

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[codspeed-hq]

Merging this PR will degrade performance by 3.17%

⚠️ 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
❌ 1 regressed benchmark
✅ 35 untouched benchmarks
⏩ 227 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	tests[eggcc-extraction]	1.4 s	1.6 s	-11.64%
⚡	Simulation	tests[eggcc-2mm]	8.7 s	8.2 s	+6.11%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing codex/inline-containers-04-infrastructure (9ee921c) with codex/inline-containers-03-sequence-benchmarks (fd5fa6f)}

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. ↩