perf: optimize date_trunc (>2x faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Resolve the date_trunc format keyword without allocating an uppercased String per row (ASCII case-insensitive match plus a last-format memo), removing a heap allocation from every row of the array-format path.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• month: 60.556% faster (base 143134ns -> cand 56457ns)
• week: 64.05% faster (base 138437ns -> cand 49768ns)
• mixed: 54.003% faster (base 157749ns -> cand 72560ns)
• year: 67.326% faster (base 134453ns -> cand 43931ns)
• quarter: 53.059% faster (base 159586ns -> cand 74911ns)

Full criterion output:

date_trunc_array_fmt/year
                        time:   [43.888 µs 43.941 µs 44.010 µs]
                        change: [−67.368% −67.326% −67.273%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  1 (1.00%) low severe
  1 (1.00%) low mild
  4 (4.00%) high mild
  7 (7.00%) high severe
date_trunc_array_fmt/quarter
                        time:   [74.951 µs 74.980 µs 75.019 µs]
                        change: [−53.137% −53.059% −52.998%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  2 (2.00%) low severe
  3 (3.00%) low mild
  2 (2.00%) high mild
  2 (2.00%) high severe
date_trunc_array_fmt/month
                        time:   [56.438 µs 56.467 µs 56.513 µs]
                        change: [−60.672% −60.556% −60.480%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) low severe
  2 (2.00%) low mild
  3 (3.00%) high mild
  3 (3.00%) high severe
date_trunc_array_fmt/week
                        time:   [49.729 µs 49.783 µs 49.882 µs]
                        change: [−64.092% −64.050% −64.005%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  4 (4.00%) high severe
date_trunc_array_fmt/mixed
                        time:   [72.540 µs 72.573 µs 72.631 µs]
                        change: [−54.045% −54.003% −53.946%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  2 (2.00%) low mild
  2 (2.00%) high mild
  3 (3.00%) high severe