Untyped code that loops over a native array (`for (const ch of checks)` in
zod, over arrays the compiled code built) paid for the loop before its
first element:
- Every check of whether a typed capsule is a generator compared its type
name against "async-generator<" with strncmp, and native array type
names also start with 'a'. Iterator creation made several such checks.
The test now looks at the second byte before comparing.
- scr_dyn_iterator called the builtin values() through the general call
path for a native array; it now creates the native iterator directly,
as it already did for untyped arrays.
- Read sites of `length` and `next` call scr_js_get_probe_native, which
answers a native array's `length` and a native iterator's `next`, where
the generic read built the array's checked view or looked the method up
by name. Other read sites keep scr_js_get_probe, so their receivers that
are not ordinary objects still miss without a further test.
