a5R 0.3.1

a5_cell() now requires hex strings to be exactly 16 characters, preventing silent construction of wrong cells from truncated input.
Improved vignettes covering the new a5_cell representation and Arrow integration.

a5R 0.3.0

a5_cell internal representation now uses 8 parallel raw byte vectors instead of hex strings, reducing memory from ~80 bytes/cell to 8 bytes/cell (#12).
a5_cell gains vec_proxy_compare() and vec_proxy_order() methods, enabling sort(), order(), unique(), duplicated(), match(), and %in% (#12).
a5_cell gains an is.na() method (#12).
New a5_cell_from_arrow() and a5_cell_to_arrow() for lossless conversion between a5_cell and Arrow uint64 arrays, enabling zero-copy Parquet I/O (#12).
New a5_u64_to_hex() and a5_hex_to_u64() for explicit conversion between a5_cell vectors and hex strings.
a5_is_cell() has been renamed to a5_is_valid() and now accepts both a5_cell vectors and character hex strings.
a5_cell_distance() and a5_cell_area() gain a units = NULL option to return plain numeric vectors without units class overhead.
New vignettes: vignette("internal-cell-representation") and vignette("arrow-parquet").