perf: optimize struct_to_json (10% faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expression.

What changes are included in this PR?

Build to_json rows directly in the StringBuilder with pre-rendered field prefixes, a right-sized buffer, and a single vectorizable column-wide escape scan replacing the per-value byte scan.

How are these changes tested?

Existing tests.

Benchmark (criterion):

• to_json: 9.92% faster (base 353519ns -> cand 318450ns)

Full criterion output:

to_json                 time:   [316.38 µs 316.78 µs 317.25 µs]
                        change: [−10.245% −9.9200% −9.6049%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  6 (6.00%) high mild