perf: Optimize floor and ceil scalar performance

[kumarUjjawal]

Which issue does this PR close?

• Part of [EPIC] Optimize performance for slow expressions datafusion-comet#2986

Rationale for this change

• The current floor and ceil implementations always convert scalar inputs to arrays via values_to_arrays(), which introduces unnecessary overhead when processing single values.

What changes are included in this PR?

• Add scalar fast path for floor and ceil
• Add criterion benchmark for floor/ceil to measure performance

Are these changes tested?

• All sqllogictest pass
• Addec benchmark

Benchmark	Array Time	Scalar Time	Speedup
floor_f64 (1024)	~396 ns	~147 ns	2.7×
ceil_f64 (1024)	~363 ns	~150 ns	2.4×
floor_f64 (4096)	~681 ns	~144 ns	4.7×
ceil_f64 (4096)	~667 ns	~144 ns	4.6×
floor_f64 (8192)	~1,638 ns	~144 ns	11.4×
ceil_f64 (8192)	~1,634 ns	~144 ns	11.3×

Are there any user-facing changes?

No

[Jefffrey]

For decimals, we could turn the scalars in arrays of size 1, pass through the existing code, then recreate a scalar from the resultant array, to benefit from code reuse and ensure we don't expand the array.

Not the cleanest way to do it but certainly the easiest.