lib/datekeys.dart exports the writer of capsules, and what its options
take: X25519Recipient and checkX25519Recipient, RandomSource and
secureRandom. The tests that imported them from their modules no longer
need to. tool/encrypt3_bench.dart times the writer on the VM and in
Node.js.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
