Validate rounds before native allocation

[OskarEichler]

Summary:

• convert rounds with NUM2UINT before allocating the output buffer
• pass the validated unsigned value to bcrypt_pbkdf
• cover values outside the native unsigned-int range

Why:
The extension currently converts rounds with NUM2ULONG and passes it to an unsigned int parameter. Values such as 2**32 + 1 silently truncate and behave like one round. Conversion also occurs after the native output allocation, complicating exceptional cleanup.

Verification:

• existing current suite: 8 runs, 31 assertions, 0 failures
• valid rounds retain identical output
• 2**32 + 1 and negative values raise RangeError instead of truncating
• native extension builds warning-free on Ruby 4.0.6

Compatibility:
Valid unsigned-int round counts are unchanged. Previously truncated out-of-range inputs are rejected explicitly.