When USE_64_BIT=1, InternalVersionT stores an int64_t, but the aarch64
NEON fast paths in scan16 (both the indexed Node48 variant and the
Node256 variant) and checkMaxBetweenExclusiveImpl(Node16) only copied the
low 32 bits of each version into uint32x4_t lanes and compared against a
truncated 32-bit readVersion. This produced wrong conflict/commit
decisions once any version exceeded 2^32 - 1.
Extract the per-version "greater than readVersion" computation into a
shared conflictMask16 helper. For 64-bit versions it uses vcgtq_s64 on
the full int64 values and narrows the resulting 16-byte mask into the
same nibble-packed layout the callers already consume. The 32-bit path is
preserved unchanged.
Closes#69
weaselbot