feat(keys): halkyon_keys, an account's keys on the device

ADR 0017's step 1, on libsodium (sodium 4.1.0+1, whose build hook
compiles the libsodium 1.0.22 it ships: a C compiler and make wherever
it builds):
- Argon2id (64 MiB, 3 passes, kept with the account and never below
  that) makes a master key, which libsodium's KDF splits into SRP's
  secret (x) and the key-encryption key.
- The account's X25519 and Ed25519 key pairs, their private halves
  wrapped with XChaCha20-Poly1305 under the KEK and under the recovery
  key, bound to their public keys.
- A 24-word BIP-39 recovery phrase (the English list, MIT) and the
  recovery key derived from it.
- Data keys sealed to the account with crypto_box_seal, bound to their
  IDs; content in ADR 0002's envelope, encrypted with a data key and
  signed with Ed25519 (detached), bound to a context the caller names.
- Secrets in libsodium's guarded memory where its API takes them.
- Tests: libsodium's Argon2id known answers, BIP-39's English vectors,
  round trips and every refusal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_keys/lib/halkyon_keys.dart

@@ -0,0 +1,23 @@
+/// An account's keys on the device (ADR 0017): what the password unlocks
+/// (Argon2id, split into SRP's x and the key-encryption key), the account's
+/// key pairs and their wrapping, the recovery phrase and its key, data keys
+/// sealed to the account, and content sealed and signed in ADR 0002's
+/// envelope. libsodium (the `sodium` package, its sumo API for Argon2id)
+/// does every primitive; nothing here is ever sent but what is wrapped,
+/// sealed or public.
+library;
+
+export 'src/account_keys.dart'
+    show AccountKeys, WrappedAccountKeys, unwrapAccountKeys, wrapAccountKeys;
+export 'src/data_keys.dart' show DataKey, openDataKey, sealDataKey;
+export 'src/key_refused.dart';
+export 'src/password_keys.dart'
+    show PasswordKdf, PasswordKeys, derivePasswordKeys, newPasswordSalt;
+export 'src/recovery_phrase.dart'
+    show
+        entropyOfRecoveryPhrase,
+        newRecoveryPhrase,
+        recoveryEntropyBytes,
+        recoveryKeyOf,
+        recoveryPhraseOf;
+export 'src/sealed_content.dart' show SealedContent, openContent, sealContent;