test(08-02): add failing RFC 7591 registration RED test in wristband

- TestPhase8RedRegistration asserts the exact public-client DCR success
  contract and fails while Server.Register is a 501 stub
- adds the Backend/Tx transaction-scoped store bundle (ClientStore,
  AuthCodeStore, RefreshTokenStore, AccessTokenIssuer) and wristband's own
  in-memory implementation for framework-level tests (D-07)
- adds crypto.go's fixed-transform helpers (random base64url, sha256 hex,
  constant-time compare, S256) and Options/Server seams for the DCR
  lifetimes, cap, sweep age and 64 KiB body bound (D-03/D-21)

wristband/crypto.go

@@ -0,0 +1,41 @@
+package wristband
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
+	"encoding/hex"
+)
+
+// randomBase64URL returns n cryptographically random bytes, base64url
+// (RawURLEncoding, no padding) encoded, matching PHP's
+// rtrim(strtr(base64_encode(random_bytes(n)), '+/', '-_'), '=') byte for
+// byte (D-01/D-04).
+func randomBase64URL(n int) (string, error) {
+	buf := make([]byte, n)
+	if _, err := rand.Read(buf); err != nil {
+		return "", err
+	}
+	return base64.RawURLEncoding.EncodeToString(buf), nil
+}
+
+// sha256Hex is the fixed transform every opaque secret (client secret, code,
+// refresh token) is compared and persisted through: never the raw variable-
+// length secret (D-04).
+func sha256Hex(raw string) string {
+	sum := sha256.Sum256([]byte(raw))
+	return hex.EncodeToString(sum[:])
+}
+
+// constantEqual compares two fixed-transform strings (sha256 hex digests or
+// PKCE S256 challenges) in constant time (D-04; RFC 7636 verifier compare).
+func constantEqual(a, b string) bool {
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}
+
+// s256Challenge is the RFC 7636 S256 transform: base64url(sha256(verifier)).
+func s256Challenge(verifier string) string {
+	sum := sha256.Sum256([]byte(verifier))
+	return base64.RawURLEncoding.EncodeToString(sum[:])
+}