Add demand-driven coarse join scheduling

[ezrosent]

Stacked on #959.

This isolates the demand-driven coarse scheduling change from the packed-executor integration:

• schedule coarse join work when it becomes available
• preserve the existing join results and packed representation
• keep the scheduling policy independently reviewable

Validation:

• cargo test -p egglog-core-relations
• cargo fmt --check
• cargo clippy -p egglog-core-relations --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.87302% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.03%. Comparing base (3cfa98a) to head (d7ea2c3).

Files with missing lines	Patch %	Lines
core-relations/src/free_join/execute.rs	68.35%	75 Missing ⚠️
concurrency/src/threadpool/mod.rs	98.71%	1 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           ezr-better-parallel-packed-trie     #967      +/-   ##
===================================================================
- Coverage                            87.13%   87.03%   -0.11%     
===================================================================
  Files                                   96       96              
  Lines                                33023    33261     +238     
===================================================================
+ Hits                                 28775    28948     +173     
- Misses                                4248     4313      +65     

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[codspeed-hq]

Merging this PR will improve performance by 5.39%

⚠️ 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[proof_testing_repro-665-set-union]	17.3 s	16.4 s	+5.39%

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/demand-driven-join-scheduling (d7ea2c3) with ezr-better-parallel-packed-trie (c7fc1d7)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 ezr-better-parallel-packed-trie (3cfa98a) during the generation of this report, so fff72bb was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩