u32 byte pack/unpack

Eight independent u32 lanes, unrolled 4x, each step unpacking 4 bytes via shift+mask, incrementing them, and repacking — no pack4x8 (or unpack4x8) builtin, just the bit-twiddling those compile to. Counted as 2 ops per lane step (one unpack + one pack of a whole u32), since the compiler folds the individual shifts and masks.

Measured in OP/s. Higher values mean better performance. Benchmark your device.