perf: optimize date_parser / cast string to date (up to 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?

Replaced the per-row chrono NaiveDate construction and duration-to-epoch-day conversion with direct integer calendar validation plus the existing days_from_civil helper, and added a branch-free fast path for the canonical yyyy-mm-dd form that skips trimming, sign and digit-count checks.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• canonical: 54.56% faster (base 124085ns -> cand 56384ns)
• mixed: 23.546% faster (base 110311ns -> cand 84337ns)

Full criterion output:

cast_string_to_date/canonical
                        time:   [56.312 µs 56.372 µs 56.441 µs]
                        change: [−54.647% −54.560% −54.464%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  1 (1.00%) low severe
  2 (2.00%) low mild
  3 (3.00%) high mild
  8 (8.00%) high severe
cast_string_to_date/mixed
                        time:   [84.211 µs 84.254 µs 84.317 µs]
                        change: [−23.818% −23.546% −23.304%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) low severe
  1 (1.00%) low mild
  1 (1.00%) high mild
  3 (3.00%) high severe