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>

pubspec.lock

@@ -747,6 +747,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.7"
+  freezed_annotation:
+    dependency: transitive
+    description:
+      name: freezed_annotation
+      sha256: "7294967ff0a6d98638e7acb774aac3af2550777accd8149c90af5b014e6d44d8"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.0"
   frontend_server_client:
     dependency: transitive
     description:
@@ -1780,6 +1788,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  sodium:
+    dependency: transitive
+    description:
+      name: sodium
+      sha256: "892b016194f60fb8f53470da866e3659877b7832b993b18ab56139a782c2d802"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.0+1"
   source_map_stack_trace:
     dependency: transitive
     description: