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/BIP39-LICENSE

@@ -0,0 +1,26 @@
+The English word list in lib/src/bip39_english.dart is BIP-39's
+(https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt), by Marek
+Palatinus, Pavol Rusnak, Aaron Voisine and Sean Bowe, 2013. BIP-39 falls under
+the MIT License:
+
+MIT License
+
+Copyright (c) 2013 Marek Palatinus, Pavol Rusnak, Aaron Voisine, Sean Bowe
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.