Keep the limiter fallback key as Data so the relay builds on Linux

SymmetricKey is not Sendable under swift-crypto on Linux, so a static of
that type is a hard error there (CI build job), while macOS CryptoKit lets it
through.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Sources/FlamenetRelay/Introductions.swift

@@ -293,13 +293,15 @@ struct HandleCertificate: Content, Equatable {
 /// random per process, which only means per-replica counters on a relay
 /// configured with neither.
 enum LimiterKeys {
-    private static let fallback = SymmetricKey(size: .bits256)
+    // Held as Data: SymmetricKey is not Sendable on Linux, so a static of that
+    // type fails to compile there (macOS builds do not catch it).
+    private static let fallbackBytes: Data = SymmetricKey(size: .bits256).withUnsafeBytes { Data($0) }
 
     static func secret(_ config: Config) -> SymmetricKey {
         if let seed = config.issuerSigningKeyRaw ?? config.federationKeyRaw, !seed.isEmpty {
             return SymmetricKey(data: SHA256.hash(data: seed + Data("limiter-keys".utf8)))
         }
-        return fallback
+        return SymmetricKey(data: fallbackBytes)
     }
 
     /// 16 hex characters standing for `parts`.