Add fresh-rematch scheduler API

[saulshanabrook]

Context

Backlog schedulers replay queued matches. Some strategies instead need to discard skipped matches and rematch the rebuilt e-graph on each scheduler iteration.

Companion experimental PR: egraphs-good/egglog-experimental#48.

Changes

• Adds FreshScheduler and registration/removal APIs.
• Separates backlog scheduler state from fresh-rematch scheduler state.
• Makes fresh schedulers query the rebuilt e-graph each step instead of replaying stale backlog.
• Adds tests covering fresh rematching, subsumed rows, and fresh scheduler removal.

Validation

• cargo test --lib scheduler
• cargo fmt
• git diff --check

[codspeed-hq]

Merging this PR will degrade performance by 5.24%

❌ 1 regressed benchmark
✅ 49 untouched benchmarks
⏩ 190 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	tests[proof_testing_eqsat-basic]	27.1 ms	28.6 ms	-5.24%

Tip

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

---

_{Comparing saulshanabrook:codex/split-scheduler-fresh-rematch (f7fd3ac) with main (8c1c70b)}

Footnotes

1. 190 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. ↩

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.30508% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.17%. Comparing base (d330f2c) to head (1b83c1f).

Files with missing lines	Patch %	Lines
src/scheduler.rs	98.30%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #883      +/-   ##
==========================================
+ Coverage   87.06%   87.17%   +0.11%     
==========================================
  Files          88       88              
  Lines       25597    25751     +154     
==========================================
+ Hits        22285    22448     +163     
+ Misses       3312     3303       -9     

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