test: add negative-verification coverage for generalized XMSS

[aryaethn]

Following up on #1.

What

test_signature_scheme_correctness covers completeness (an honest signature verifies) and the SSZ tests cover malformed bytes, but nothing asserts that verify rejects a well-formed signature presented against the wrong inputs — the properties the scheme's unforgeability rests on. This adds that adversarial counterpart.

• test_signature_scheme_robustness<T: SignatureScheme> — a generic template mirroring the existing correctness one, asserting verify rejects a wrong message, a wrong (in-lifetime) epoch, and a wrong public key. Wired into the Poseidon-W1 and aborting-hypercube instantiations.
• test_verify_rejects_tampered_signature — swapping two chain hashes yields a well-formed but invalid signature that must be rejected.
• proptest_verify_rejects_wrong_message — fuzzes message binding.

Tests only — no production code changes.

Tested

$ cargo test --lib generalized_xmss::tests
test result: ok. 15 passed; 0 failed

$ cargo test --lib generalized_xmss::tests::test_verify_rejects_tampered_signature
test result: ok. 1 passed; 0 failed

$ cargo test --lib generalized_xmss::tests::proptest_verify_rejects_wrong_message
test result: ok. 1 passed; 0 failed

$ cargo clippy --all-targets --all-features -- -D warnings
Finished

Refs #1.