sercon v0.5.16

Adds the PGP backend to api.encrypt, completing the
two-backend design detectBackend (v0.5.8) anticipated. encrypt
/ decrypt now auto-dispatch between age and PGP based on the
key/ciphertext format — one API, two backends. New dependency:
github.com/ProtonMail/go-crypto/openpgp (the maintained pure-Go
PGP fork; x/crypto/openpgp is frozen).

Added

• api.encrypt.keygenPgp(opts?) → armored { publicKey, privateKey }
  PGP key blocks (RSA 2048). opts.name / opts.email populate the
  primary user ID.
• api.encrypt.encrypt routes to PGP when the first recipient is a
  PGP public-key block; output is always ASCII-armored
  (-----BEGIN PGP MESSAGE-----). Multi-recipient works the same as
  age (any recipient's private key decrypts).
• api.encrypt.decrypt routes to PGP when an identity is a PGP
  private-key block or the ciphertext is an armored PGP message.
  Accepts armored or binary PGP ciphertext.
• age and PGP recipient sets can't be mixed in one call (the
  formats are incompatible) — the backend is picked from the first
  recipient / the identity / the ciphertext.
• A deliberately small PGP subset: keygen + encrypt + decrypt. No
  signing, subkey management, or web-of-trust.
• TestEncrypt_PGPRoundTrip, TestEncrypt_PGPDetectBackend,
  TestEncrypt_PGPWrongKeyThrows, TestEncrypt_AgeAndPGPDontCross
  (regression guard that the two backends stay independent).
• examples/scripts/encrypt.ts grows a PGP section; MANUAL §5
  rewritten to describe the two-backend dispatch + keygenPgp.

Changed

• OUT-OF-SCOPE.md's PGP-backend entry resolved.