Reject a BitArray whose total bit length is not a multiple of 8.

yabase encoders walk their input one byte at a time. A sub-byte trailing segment (<<1:size(1)>>, <<0xFF, 0x80, 1:size(3)>>, …) would otherwise be silently dropped by the byte-walker pattern, producing a string that decodes to a strictly shorter BitArray than the caller supplied — silent data loss. Every encode function calls this at the boundary so the truncation becomes a loud crash instead.

Crashing rather than returning a Result keeps the public encode shape per-codec compatible with v0.14.x callers (fn(BitArray) -> String); a sub-byte input has always been outside the contract, so there is no caller that previously “worked” with one. RFC 4648-conformant encoders never receive sub-byte input in practice, so the guard is a safety net for programmer-error inputs only.