perf: reduce heap allocations using SmallVec for RowView values

[Einswilli]

Ryx targets 1-5 microseconds overhead. Currently, RowView stores row values in a Vec, which always allocates on the heap. For queries with a small number of columns (the majority of cases), this is unnecessary overhead.

Goal: Replace Vec<T> with SmallVec<[T; N]> in the RowView structure to enable stack allocation for rows with up to N columns.

Implementation hint:

1. Add the smallvec crate to ryx-backend.
2. Modify the RowView definition and the decode_rows logic to use SmallVec.
3. Run benchmarks to quantify the reduction in allocator pressure.