perf: optimize cast_decimal128_to_utf8 in datafusion-comet-spark-expr

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Replaced the per-row u128 to_string heap allocation, format! machinery, and "0".repeat in the LEGACY Decimal128-to-string cast with an allocation-free stack digit buffer that renders the coefficient in 19-digit 64-bit chunks and appends slices directly to the reused output buffer.

How are these changes tested?

Existing tests + expanded unit tests.

Benchmark (criterion):

• decimal128_leading_zeroes: 73.764% faster (base 197033ns -> cand 51693ns)
• decimal128_wide: 20.876% faster (base 198585ns -> cand 157128ns)
• decimal128_scientific: 49.361% faster (base 172590ns -> cand 87397ns)
• decimal128_scale_2: 52.624% faster (base 155170ns -> cand 73513ns)

Full criterion output:

cast_decimal_to_string/decimal128_scale_2
                        time:   [73.783 µs 73.913 µs 74.059 µs]
                        change: [−52.799% −52.624% −52.452%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild
cast_decimal_to_string/decimal128_leading_zeroes
                        time:   [51.381 µs 51.495 µs 51.615 µs]
                        change: [−73.817% −73.764% −73.700%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 22 outliers among 100 measurements (22.00%)
  16 (16.00%) low severe
  1 (1.00%) low mild
  3 (3.00%) high mild
  2 (2.00%) high severe
cast_decimal_to_string/decimal128_wide
                        time:   [157.06 µs 157.33 µs 157.66 µs]
                        change: [−20.987% −20.876% −20.752%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) high mild
  3 (3.00%) high severe
cast_decimal_to_string/decimal128_scientific
                        time:   [87.563 µs 87.699 µs 87.832 µs]
                        change: [−49.456% −49.361% −49.272%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) low severe
  1 (1.00%) high mild