perf: optimize spark_cast_float64_to_utf8 (~40% faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Format float-to-string casts straight into a GenericStringBuilder with a reused scratch buffer instead of allocating a String per row (and a second one in the scientific-notation path), removing one malloc/free and one copy per value.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• cast_f32_to_utf8: 42.453% faster (base 408097ns -> cand 234847ns)
• cast_f64_to_utf8: 41.57% faster (base 418096ns -> cand 244294ns)

Full criterion output:

cast_float_to_string/cast_f64_to_utf8
                        time:   [243.85 µs 244.03 µs 244.23 µs]
                        change: [−41.767% −41.570% −41.425%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe
cast_float_to_string/cast_f32_to_utf8
                        time:   [234.71 µs 234.86 µs 235.08 µs]
                        change: [−42.515% −42.453% −42.390%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) low mild
  1 (1.00%) high mild
  3 (3.00%) high severe

[mbutrovich]

First pass, thanks @andygrove!

[mbutrovich]

GenericStringBuilder::<OffsetSize>::with_capacity(array.len(), array.len() * 8). Eight bytes
per value under-provisions typical fractional output like 1234.5678 (9 bytes) or the
scientific forms this cast produces (4.9E-324 is 8, -1.4E-45 is 8, longer with more
mantissa digits), so the value buffer will still reallocate on realistic data. arrow-rs itself
uses an AVERAGE_STRING_LENGTH of 16 for its own capacity helper
(arrow-array/src/builder/generic_bytes_builder.rs:376).

Suggested change: raise the hint to array.len() * 16 to match arrow-rs's own average and
avoid mid-loop growth on ordinary float text. The cost of a slightly large preallocation is
negligible per that same arrow-rs doc note.