feat(crypto): SRP-6a, the register's half and the device's

ADR 0017's step 1. Generic over group and hash (SrpGroup); production
takes RFC 5054's 3072-bit group with SHA-256. k = H(N | PAD(g)),
u = H(PAD(A) | PAD(B)), K = H(PAD(S)),
M1 = H(H(N) xor H(g) | H(I) | s | PAD(A) | PAD(B) | K),
M2 = H(PAD(A) | M1 | K). A and B must lie in 1..N-1 and u must not be
0; a and b are 256 bits from Random.secure(); proofs are compared in
constant time. x comes from the caller (halkyon_keys makes it); RFC
5054's own x is there for the vectors only.

- The register's half in halkyon_base_server (base.dart: srpChallenge,
  srpVerify), the device's written by hand in halkyon_base_client
  (srp.dart: SrpClient, srpVerifier). Serverpod's generator cleans and
  writes only a client's lib/src/protocol/ (serverpod_cli 4.0.4's
  _getDirectoriesRequiringCleaning; a generate leaves srp/ untouched).
- Tests: RFC 5054 Appendix B on both halves; the client against the
  register, succeeding and every refusal.
- The module's tests depend on its client, and pub resolves every
  member's dev dependencies, so Learn's server image takes the module's
  client into its workspace too.
- The format check covers the hand-written half.

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_base_server/pubspec.yaml

@@ -21,9 +21,13 @@ dependencies:
   http: ^1.6.0
   # Mails email sign-in's codes over SMTP (Halkyon Learn's ADR 0010).
   mailer: ^7.2.0
+  meta: ^1.16.0
   serverpod: 4.0.4
   serverpod_auth_idp_server: 4.0.4
 
 dev_dependencies:
+  # SRP's client half, to hold the register's half to it (test/unit/srp/).
+  halkyon_base_client:
+    path: ../halkyon_base_client
   serverpod_test: 4.0.4
   test: ^1.25.5