perf: optimize spark_ceil (3x faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Decimal ceil now uses 64-bit hardware division for values/divisors that fit in i64 instead of the 128-bit division intrinsic, and the per-element rounding closure is monomorphized rather than called through a &dyn Fn vtable.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• ceil_decimal_18_4: 68.795% faster (base 23908ns -> cand 7460ns)
• ceil_decimal_38_6: -2.613% faster (base 37323ns -> cand 38298ns)

Full criterion output:

ceil/ceil_decimal_18_4  time:   [7.3997 µs 7.4042 µs 7.4101 µs]
                        change: [−68.905% −68.795% −68.675%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 16 outliers among 100 measurements (16.00%)
  4 (4.00%) high mild
  12 (12.00%) high severe
ceil/ceil_decimal_38_6  time:   [38.243 µs 38.268 µs 38.302 µs]
                        change: [+2.4580% +2.6128% +2.7361%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

[mbutrovich]

First pass, thanks @andygrove!

[mbutrovich]

ceil.rs:97-254 tests only small unscaled values (max 12999), which all take the new i64 fast path. The entire point of this PR is the branch at ceil.rs:85, and the fallback arm (_ => div_ceil(x, div) at ceil.rs:93) has zero direct coverage. A regression that broke only the wide path would pass CI.

Suggested change: add an array test with a Decimal128(38, s) input whose unscaled values exceed i64::MAX (positive and negative), asserting the ceil result. Example: unscaled 20_000_000_000_000_000_000 (> i64::MAX) at scale 6 targeting Decimal128(38, 0), plus a negative sibling and an exact multiple, so both the fast and fallback arms plus the negative-remainder branch are covered in one suite.

[mbutrovich]

utils.rs:60 now takes f: impl Fn(i128) -> i128, but the array call site at ceil.rs:51 still passes &f. This compiles because &F: Fn via the blanket impl, so F monomorphizes to &closure and dispatch is static, but it is now a stray reference that obscures the intent of the change and adds a pointer indirection the PR was trying to remove.

Suggested change: pass the closure by value at ceil.rs:51: make_decimal_array(array, precision, scale, f).

[mbutrovich]

utils.rs:49 still declares f: &dyn Fn(i128) -> i128 and ceil.rs:68 passes &f. The PR's stated rationale is "the per-element rounding closure is monomorphized rather than called through a &dyn Fn vtable," but that only happened for the array path. The scalar path is a single element so the perf cost is irrelevant, however the two helpers now have inconsistent signatures for no reason, which is a maintenance trap.

Suggested change: change make_decimal_scalar at utils.rs:49 to f: impl Fn(i128) -> i128 and pass f by value at ceil.rs:68, matching make_decimal_array. This keeps the two helpers symmetric.