Expression enum and Root variant

[mhk197]

Makes Root an Expression variant instead of a ScalarFnVTable implementation.

This is groundwork for lambdas and variables — those are also nodes whose dtype comes from the
scope rather than from their children, and they need the same treatment.

Root is not a scalar function. Its dtype comes from the scope rather than from its children (it has no children) and it is not executable.

This PR makes Expression an enum instead of a struct. Root becomes a variant, and upcoming Lambda and Variable become variants as well.

pub enum Expression {
    Scalar { scalar_fn: ScalarFnRef, children: Arc<Vec<Expression>> },
    Root,
}

This is a small breaking change.

[codspeed-hq]

Merging this PR will degrade performance by 10.68%

⚠️ 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 regressed benchmark
✅ 1928 untouched benchmarks
⏩ 51 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	compress_fsst[(10000, 64, 8)]	9.6 ms	10.7 ms	-10.68%

Tip

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

---

_{Comparing mk/root-expression (8bd1b1f) with develop (4e3be5b)}

Footnotes

1. 51 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. ↩