fix: PartitionOptions hash_rate upper bound to match u8 storage

[VolodymyrBg]

• Problem: The PartitionOptions.hash_rate field is stored as u8 but the constructor allowed values up to 256. Passing 256 truncates to 0 when cast to u8, is serialized as 0, and on deserialization becomes 0, which violates the invariant (>= 1) and can trigger a panic in PartitionOptions::new. The docstring for with_partitions also incorrectly stated the upper bound as 256.
• Why it’s a problem: u8 cannot represent 256; truncation silently corrupts the value, causing inconsistent serialization/deserialization behavior and potential runtime panics. This also breaks expectations for stable encoding of options.
• Fix: Tighten the hash_rate upper bound to 255 in PartitionOptions::new and update the with_partitions documentation accordingly. This prevents truncation, ensures values round-trip correctly via write_u8/read_u8, and keeps binary compatibility without widening the serialized type.

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