fix: return Ed25519 verify result from Identity::validate

[SAY-5]

Summary

Identity::validate called _sig_pub->verify(signature, message) but discarded its
bool return and unconditionally returned true on the no-exception path. Since
Ed25519PublicKey::verify returns false on a signature mismatch rather than throwing,
any non-matching signature was accepted as valid. This returns the verify result directly.

Fixes #51.

Test

Added testIdentityValidate to test_crypto: signs a message and checks that the valid
signature passes, while a signature with a flipped byte and a valid signature against a
different message both fail. The new test fails on master and passes with this change
(pio test -e native17 -f test_crypto).

[attermann]

Thank you @SAY-5 for the fix!