perf: optimize spark_floor (up to 4x faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize an existing expression.

What changes are included in this PR?

Decimal floor now monomorphizes the per-element op (was &dyn Fn) and dispatches on input scale to a const divisor with an i64 fast path, so the per-row 128-bit __divti3 division libcall becomes a multiply-and-shift.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• spark_floor_ float64: 10.548% faster (base 1525ns -> cand 1364ns)
• spark_floor_ decimal128(38,6) wide values: 3.043% faster (base 38997ns -> cand 37810ns)
• spark_floor_ decimal128(38,2): 69.852% faster (base 25462ns -> cand 7676ns)
• spark_floor_ decimal128(38,18): 75.222% faster (base 28693ns -> cand 7109ns)

Full criterion output:

spark_floor: decimal128(38,2)
                        time:   [7.6379 µs 7.6620 µs 7.6869 µs]
                        change: [−69.961% −69.852% −69.739%] (p = 0.00 < 0.05)
                        Performance has improved.

spark_floor: decimal128(38,18)
                        time:   [7.0976 µs 7.1216 µs 7.1451 µs]
                        change: [−75.295% −75.222% −75.141%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) low mild

spark_floor: decimal128(38,6) wide values
                        time:   [37.480 µs 37.607 µs 37.739 µs]
                        change: [−3.4647% −3.0427% −2.6266%] (p = 0.00 < 0.05)
                        Performance has improved.

spark_floor: float64    time:   [1.3466 µs 1.3669 µs 1.3899 µs]
                        change: [−12.450% −10.548% −8.3898%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  7 (7.00%) high mild
  5 (5.00%) high severe