fix(spec): reserve 16 bytes for non-compact decimal in BinaryRow

[jackylee-ch]

write_decimal_var_len pads the variable-length part to a multiple of 8, but Java
reserves 16 bytes for every non-compact Decimal — AbstractBinaryWriter#writeDecimal
does ensureCapacity(16), zeroes both halves, and advances the cursor by 16
unconditionally.

So whenever the minimal form fits in 8 bytes (|unscaled| < 2^63, e.g.
DECIMAL(38, 18) holding 1.5) Rust writes a row 8 bytes shorter than Java's.
Decoding is unaffected — both sides read the stored (offset, size) — but
hash_code hashes the whole buffer and feeds default_bucket, so the row lands in
a different bucket than in Java.

The existing test uses 1e19, whose 9-byte minimal form already rounded up to 16,
so this case had no coverage.