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/server.go

@@ -14,6 +14,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"net/http"
+	"time"
 )
 
 // Options configures a Server's advertised endpoints and metadata values.
@@ -42,6 +43,20 @@ type Options struct {
 	// AuthorizationResponseIssParameterSupported is the RFC 9207 metadata
 	// capability flag. PHP default: true.
 	AuthorizationResponseIssParameterSupported bool
+
+	// DCRClientCap is the maximum number of unrevoked OAuth clients RFC
+	// 7591 registration allows (PHP OAuthRegisterController::MAX_CLIENTS).
+	// PHP default: 200 (D-03).
+	DCRClientCap int
+
+	// DCRUnconsentedSweepAge is how old an unconsented, dynamically
+	// registered client (non-nil RegistrationIP) must be before
+	// registration sweeps it. PHP default: 24h (D-03).
+	DCRUnconsentedSweepAge time.Duration
+
+	// RegisterMaxBodyBytes bounds the RFC 7591 registration request body
+	// before JSON decoding (D-21, T-08-DCR-FLOOD). PHP default: 65536 (64 KiB).
+	RegisterMaxBodyBytes int64
 }
 
 // DefaultOptions returns PHP-parity defaults for every metadata option
@@ -52,18 +67,41 @@ func DefaultOptions() Options {
 		ScopesSupported:                            []string{"read", "write", "ai", "offline_access"},
 		TokenEndpointAuthMethodsSupported:          []string{"none", "client_secret_post", "client_secret_basic"},
 		AuthorizationResponseIssParameterSupported: true,
+		DCRClientCap:                               200,
+		DCRUnconsentedSweepAge:                     24 * time.Hour,
+		RegisterMaxBodyBytes:                       65536,
 	}
 }
 
 // Server is the app-agnostic wristband authorization-server surface. It is
 // constructed with Options and never imports an application package.
 type Server struct {
-	opts Options
+	opts    Options
+	backend Backend
+
+	// now and randomBytes are deterministic clock/entropy seams so tests
+	// can control timestamps and generated secrets without depending on
+	// wall-clock time or true randomness (08-02-PLAN.md Task 2).
+	now         func() time.Time
+	randomBytes func(n int) (string, error)
 }
 
-// NewServer constructs a Server from Options.
+// NewServer constructs a Server from Options. The backend is nil until
+// SetBackend is called (D-09: the metadata route needs no backend at all,
+// so plugin boot can construct a Server before a *gorm.DB is available).
 func NewServer(opts Options) *Server {
-	return &Server{opts: opts}
+	return &Server{
+		opts:        opts,
+		now:         time.Now,
+		randomBytes: randomBase64URL,
+	}
+}
+
+// SetBackend attaches the app's transaction-scoped store bundle. Handlers
+// that need persistence (Register) return an opaque 500 until this is
+// called.
+func (s *Server) SetBackend(b Backend) {
+	s.backend = b
 }
 
 // metadataDocument is the exact unwrapped RFC 8414 body. Field order matches