fix: align hashing, tidy pow integer exponents

[cheehenn]

This PR improves numeric correctness and cleans up the implementation
details around hashing and exponentiation.

Changes:

• Align Hash behavior with canonical equality so values that compare
  equal hash consistently.
• Rework pow integer-exponent handling (including negative exponents
  and signed zero edge cases) and use a single exponentiation-by-
  squaring path.
• Address clippy findings and apply cargo fmt formatting.

Validation:

• cargo test --all-features
• cargo fmt --all --check
• cargo clippy --all-features -- -D warnings