More efficient key sign lookups

[djberg96]

The issue is that Key.find is expensive (~7ms per call) because it creates a new GPGME context each time. When using encrypt(..., sign: true, signers: ...), the code was making two separate Key.find calls, one for recipients, and one for signers. This doubled the key lookup overhead, making encrypt+sign with explicit signers significantly slower.

Performance Improvement: I added a resolve_keys_for_signing helper method in crypto.rb that reuses already-fetched recipient Key objects when the signer matches a recipient (by fingerprint, key ID, or email), and only calls Key.find for signers that aren't already in the recipient list.

In local benchmarks, the explicit signer case went from being 78% slower than encrypt-only to only 24% slower, which is now in line with the expected overhead of signing.

Addresses #156

[ueno]

This looks great; thank you also for adding the benchmark script!