The previous byte-position test packed with the test's own packer on
both sides of the comparison, so it exercised none of the library code.
It now pins hand-computed byte values (43/100/127/42/124...) at the
region boundaries (79/80, 119/120) as ground truth and drives both
directions through the library: to_float must place each literal byte's
trit at the exact element, and from_float must produce the exact literal
byte, for both planes.
Also probes ggml_validate_row_data over all 256 byte values in qs and
qh positions (must accept exactly the 243/81 reachable codes), the
all-0xaa block, and a well-formed packed block.