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>

.dockerignore

@@ -1,6 +1,7 @@
 # The Docker build context is the repository root. Nothing secret, local or bulky
 # goes into it: Learn's server's and curator's Dockerfiles copy their whole package
-# folders (and the server the module it builds on).
+# folders (and the server the module it builds on, with the module's client, which the
+# module's tests depend on).
 
 # Secrets and local state
 **/config/passwords.yaml
@@ -26,7 +27,6 @@ apps/learn/halkyon_learn_client/
 apps/learn/tool/
 apps/todo/
 packages/halkyon_app/
-packages/halkyon_base_client/
 packages/halkyon_core/
 packages/halkyon_interop/
 packages/halkyon_ui/