Add primitive body typecheck helpers

[saulshanabrook]

Context

This is the egglog-side support needed by egglog-experimental's body-defined (primitive ...) command. It intentionally contains only the reusable API surface and not the experimental command implementation.

Companion experimental PR: egraphs-good/egglog-experimental#46.

Changes

• Adds output-constrained expression typechecking helpers on EGraph.
• Adds SpecializedPrimitive::apply for evaluating an already-specialized primitive against runtime values.
• Adds focused tests and an unreleased changelog entry.

Validation

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

[codspeed-hq]

Merging this PR will improve performance by 27.82%

⚠️ 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

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 22 untouched benchmarks
🆕 1 new benchmark
⏩ 215 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
🆕	Simulation	tests[repro-herbie-vanilla]	N/A	1.4 s	N/A
❌	Simulation	tests[python_array_optimize]	1.5 s	1.6 s	-6.94%
⚡	Simulation	rust_rule_fib[rule_run_1000]	280.7 ms	202.7 ms	+38.46%
⚡	Simulation	tests[cykjson]	440.2 ms	271.6 ms	+62.08%

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-primitive-body-runtime-apis (f8883c7) with main (8c1c70b)²}

Footnotes

1. 215 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 main (36de434) during the generation of this report, so 8c1c70b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.99%. Comparing base (36de434) to head (f8883c7).

Files with missing lines	Patch %	Lines
src/exec_state.rs	0.00%	51 Missing ⚠️
src/lib.rs	95.00%	7 Missing ⚠️
src/typechecking.rs	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
- Coverage   87.06%   86.99%   -0.08%     
==========================================
  Files          88       88              
  Lines       25844    26064     +220     
==========================================
+ Hits        22502    22675     +173     
- Misses       3342     3389      +47     

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