feat(08-01): implement exact RFC 8414 metadata writer in wristband

- Server.Metadata now writes the unwrapped 11-field PHP-parity document
  through a local no-envelope, no-trailing-newline JSON writer with the
  PHP Cache-Control: no-cache, private header (D-06); response types,
  grant types and PKCE method stay fixed protocol constants
- TestPhase8RedMetadata now passes; TestMetadataExactBytes and
  TestMetadataUsesConfiguredOptions cover byte-exact output and the four
  configurable Options fields

wristband/server_test.go

@@ -1,8 +1,10 @@
 package wristband
 
 import (
+	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 )
 
@@ -37,3 +39,85 @@ func TestPhase8RedMetadata(t *testing.T) {
 		t.Fatalf("PHASE8_RED:metadata: Cache-Control = %q, want \"no-cache, private\"", cc)
 	}
 }
+
+// TestMetadataExactBytes is the GREEN direct-handler regression for
+// TestPhase8RedMetadata: same PHP-fixture bytes, now expected to pass.
+func TestMetadataExactBytes(t *testing.T) {
+	opts := DefaultOptions()
+	opts.Issuer = "https://plytarium.com"
+	srv := NewServer(opts)
+
+	req := httptest.NewRequest(http.MethodGet, "/.well-known/oauth-authorization-server", nil)
+	req.Header.Set("Accept", "application/json")
+	rec := httptest.NewRecorder()
+	srv.Metadata(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	const want = `{"issuer":"https://plytarium.com","authorization_endpoint":"https://plytarium.com/oauth/mcp/authorize","token_endpoint":"https://plytarium.com/oauth/mcp/token","registration_endpoint":"https://plytarium.com/oauth/mcp/register","response_types_supported":["code"],"grant_types_supported":["authorization_code","refresh_token"],"code_challenge_methods_supported":["S256"],"token_endpoint_auth_methods_supported":["none","client_secret_post","client_secret_basic"],"scopes_supported":["read","write","ai","offline_access"],"service_documentation":"https://plytarium.com/help","authorization_response_iss_parameter_supported":true}`
+
+	if got := rec.Body.String(); got != want {
+		t.Fatalf("body mismatch\n got:  %s\nwant: %s", got, want)
+	}
+	if strings.HasSuffix(rec.Body.String(), "\n") {
+		t.Fatal("body has a trailing newline, want none")
+	}
+	if _, hasData := decodeAsMap(t, rec.Body.Bytes())["data"]; hasData {
+		t.Fatal("body has a house \"data\" envelope, want unwrapped RFC 8414 document")
+	}
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Fatalf("Content-Type = %q, want application/json", ct)
+	}
+	if cc := rec.Header().Get("Cache-Control"); cc != "no-cache, private" {
+		t.Fatalf("Cache-Control = %q, want %q", cc, "no-cache, private")
+	}
+}
+
+// TestMetadataUsesConfiguredOptions proves service_documentation,
+// scopes_supported and the auth-methods list come from Options, not a
+// hardcoded literal, while the three protocol-constant fields never change
+// (D-06: only those four fields are configurable).
+func TestMetadataUsesConfiguredOptions(t *testing.T) {
+	opts := Options{
+		Issuer:                            "https://example.test",
+		ServiceDocumentationPath:          "/docs/oauth",
+		ScopesSupported:                   []string{"read"},
+		TokenEndpointAuthMethodsSupported: []string{"client_secret_post"},
+		AuthorizationResponseIssParameterSupported: false,
+	}
+	srv := NewServer(opts)
+
+	rec := httptest.NewRecorder()
+	srv.Metadata(rec, httptest.NewRequest(http.MethodGet, "/.well-known/oauth-authorization-server", nil))
+
+	var got map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+	if got["service_documentation"] != "https://example.test/docs/oauth" {
+		t.Fatalf("service_documentation = %v, want configured path", got["service_documentation"])
+	}
+	if scopes, _ := got["scopes_supported"].([]any); len(scopes) != 1 || scopes[0] != "read" {
+		t.Fatalf("scopes_supported = %v, want [read]", got["scopes_supported"])
+	}
+	if methods, _ := got["token_endpoint_auth_methods_supported"].([]any); len(methods) != 1 || methods[0] != "client_secret_post" {
+		t.Fatalf("token_endpoint_auth_methods_supported = %v, want [client_secret_post]", got["token_endpoint_auth_methods_supported"])
+	}
+	if got["authorization_response_iss_parameter_supported"] != false {
+		t.Fatalf("authorization_response_iss_parameter_supported = %v, want false", got["authorization_response_iss_parameter_supported"])
+	}
+	if rts, _ := got["response_types_supported"].([]any); len(rts) != 1 || rts[0] != "code" {
+		t.Fatalf("response_types_supported = %v, want the fixed [code] constant", got["response_types_supported"])
+	}
+}
+
+func decodeAsMap(t *testing.T, body []byte) map[string]any {
+	t.Helper()
+	var m map[string]any
+	if err := json.Unmarshal(body, &m); err != nil {
+		t.Fatalf("decode response: %v", err)
+	}
+	return m
+}