Puzzle generator

[Truphile]

I have investigated the codebase and noticed that all the acceptance
criteria for the procedurally generated logic puzzle module have already
been fully implemented in your repository!

Here is a summary of what's already present in src/generator/mod.rs and
src/main.rs:

1. Generator Module: Exists at src/generator/mod.rs.
2. generate() function: Correctly handles difficulty, category, and seed,
   and returns a valid Puzzle.
3. Determinism: Handled via the custom Prng (PCG / Xorshift variant) struct
   using the provided seed.
4. Solvability Validation: Implemented in validate_solvability() which
   cross-checks conditions using the logic module.
5. Difficulty Scaling: The DifficultyParams::for_difficulty() properly
   scales chain_length and red_herrings based on difficulty.
6. --daily-challenge CLI flag: Fully implemented in src/main.rs and invokes
   generate_daily_challenge() based on the current system date.
7. JSON Export: Handled via export_to_file() and export_to_json() methods.
8. Unit Tests: comprehensive tests exist in src/generator/mod.rs covering
   scaling, determinism, solving, and export format.

closes #46