[LOW] Invalidate cached ECDSA keys after point mutation

[OskarEichler]

Summary

Invalidate ECDSA::VerifyKey’s cached OpenSSL public-key wrapper after delegated in-place point mutations. Verification and serialization then use the key state exposed by the mutable point object.

Reproduction

After serialize or verify initializes @ec_key, calls such as invert! mutate the delegated EC point but leave subsequent serialization and verification on the old cached point. A verifier can therefore authenticate with a stale key while exposing a different current point to callers.

Verification

• Focused model fails on master and confirms candidate serialization changes after point inversion
• Existing suite: 31 examples, 0 failures on Ruby 4.0.6 through rbenv
• The cumulative six-check model and RuboCop (20 files) pass
• Unmutated verification keys retain the cached fast path; mutation methods preserve underlying return values
• No repository tests or dependency files changed

Compatibility and breaking changes

Unmutated keys are unchanged. Code that mutates a verification point after warming its cache now verifies and serializes the mutated key instead of the stale pre-mutation key; relying on the stale behavior was unsafe.