parse_structural_simd
def parse_structural_simd(input: String) -> StructuralIndexBuild a structural index using a branchless SIMD scan.
Output is byte-for-byte identical to
stage1_scalar.parse_structural_scalar(input) -- the equivalence is
enforced by tests/test_stage1_equivalence.mojo, including a
full-document run against the benchmark corpora.
The expensive per-marker questions ("is this \ an escape,
is this " already escaped, is this , inside a string?")
become branchless bit-twiddling on the per-chunk uint64 masks,
leaving just one loop -- iterating set bits in the final emit
mask with count_trailing_zeros.