refactor(core,keys): BIP-39's English list moves into halkyon_core

The list a recovery phrase's words are of moves from halkyon_keys into
halkyon_core, exported as bip39English, so that the server module can
take a waiting account's matching code from it (Halkyon Learn's ADR
0021, D33) without depending on halkyon_keys and libsodium. Its test
(2048 words, sorted, the published file's SHA-256, unique by their first
four letters) moves along; its MIT licence too.

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_core/test/bip39_english_test.dart

@@ -0,0 +1,26 @@
+import 'package:crypto/crypto.dart';
+import 'package:halkyon_core/halkyon_core.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("the word list is BIP-39's English list: 2,048 words, sorted, each "
+      'once', () {
+    expect(bip39English, hasLength(2048));
+    expect(bip39English.toSet(), hasLength(2048));
+    expect([...bip39English]..sort(), bip39English);
+    // The SHA-256 of english.txt, a word a line.
+    expect(
+      sha256.convert('${bip39English.join('\n')}\n'.codeUnits).toString(),
+      '2f5eed53a4727b4bf8880d8f3f199efc90e58503646d9ff8eff3a2ed3b24dbda',
+    );
+  });
+
+  test('no two words begin with the same four letters', () {
+    expect(
+      bip39English
+          .map((word) => word.substring(0, word.length.clamp(0, 4)))
+          .toSet(),
+      hasLength(2048),
+    );
+  });
+}