#7 add SCRAM-SHA-512 and document TLS 1.3 support

- Implement SCRAM-SHA-512 (RFC 5802 with SHA-512) as a Luma-side
  SaslMechanism and register it ahead of Martin's SHA-256/SHA-1/PLAIN,
  so modern servers preferring SHA-512 authenticate with it.
- Verify the math against Python-computed reference vectors (salted
  password, client proof, server signature) in SCRAMSHA512Tests and
  guard the mechanism registration in Scripts/verify.sh.
- Confirm the TLS stack negotiates TLS 1.3 (negotiatedSSL + ALPN via
  SecureTransport, handshake verified against a TLS 1.3-only server)
  and document the TLS/SCRAM posture in SECURITY.md.

Sources/Shared/UI/LoginView.swift

@@ -79,9 +79,9 @@ struct LoginView: View {
                 TextField("you@example.org", text: $jid)
                     #if os(iOS)
                         .keyboardType(.emailAddress)
+                        .textInputAutocapitalization(.never)
                     #endif
                     .textContentType(.username)
-                    .textInputAutocapitalization(.never)
                     .autocorrectionDisabled()
                     .textFieldStyle(.roundedBorder)
                     .disableAutocorrection(true)
@@ -101,7 +101,9 @@ struct LoginView: View {
                         }
                     }
                     .autocorrectionDisabled()
+                    #if os(iOS)
                     .textInputAutocapitalization(.never)
+                    #endif
                     .textFieldStyle(.roundedBorder)
 
                     Button {