feat: add seeded model initialization

[kholdrex]

Summary

Adds reproducible model initialization for IrtRuby models through an optional seed: keyword. The default constructor behavior remains unchanged, while seeded constructors use instance-local random generators so reproducible runs do not disturb Ruby's global random number generator.

Changes

• Adds seed: support to Rasch, 2PL, and 3PL model constructors.
• Uses seeded instance-local randomness for initial abilities, difficulties, discriminations, and guessings when requested.
• Adds shared regression coverage for identical seeded results, different initial states for different seeds, and global RNG preservation.
• Documents reproducible initialization in the README.

Test plan

• RSpec suite passed.
• RuboCop passed.
• Gem build passed.
• Diff whitespace check passed.