perf: collect nested struct addresses once in field-major append

[andygrove]

Summary

• In append_struct_fields_field_major, the first pass now collects nested struct addresses and sizes alongside the null bitmap
• The per-field second pass uses these pre-collected addresses via point_to() instead of re-reading from parent row pointer arrays (read_row_at!) and calling get_struct() for every field of every row
• Same optimization applied to the Binary, Utf8, Decimal128, nested Struct, and List/Map field cases

Rationale

Previously, for a struct with F fields and N rows, the code performed NF pointer dereferences into the parent row address/size arrays plus NF get_struct() calls (each involving get_offset_and_len which reads an i64 and does bit manipulation). After this change, parent row reads and get_struct calls happen only N times total in the first pass, and the second pass uses cheap point_to() calls with the cached addresses.

Test plan

• cargo clippy --all-targets --workspace -- -D warnings passes
• Existing struct row-to-columnar tests cover these code paths