fix: bind prepared cache to setup generators

[0xAndoroid]

Summary

• bind prepared G1/G2 points to their source generator values, not only vector lengths
• validate the cached prefix before every setup-specialized pairing; mismatches fall back to uncached preparation
• reinitialize equal-length caches when generator values differ while retaining matching-prefix reuse
• remove correctness dependence on cache invalidation during setup creation and persistence

Failure mode

init_cache previously treated any cached vectors of sufficient length as reusable. multi_pair_g1_setup and multi_pair_g2_setup then consumed those prepared points without checking the generator arguments. Two random setups of the same size therefore produced silently incorrect pairings.

Regression coverage

• equal-length setup replacement
• cached G1 and G2 pairing fallback for a different setup
• matching-prefix cache reuse

Verification

• cargo nextest run --all-features --cargo-quiet
• cargo clippy --all-features -q --all-targets --message-format=short -- -D warnings
• cargo doc --all-features -q --message-format=short --no-deps
• cargo fmt -q --message-format=short