Establish watch relay contracts and platform package wiring

Adapters/WatchBridge/Sources/Common/RelayContainer.swift

@@ -0,0 +1,15 @@
+import Foundation
+import RemoteCore
+
+/// Typed outgoing container seam. Workstream B owns WCSession dictionary parsing,
+/// strict incoming key/type validation, routing and exactly-once replies.
+public struct RelayContainer: Sendable {
+    public let schemaVersion = RelayWire.schemaVersion
+    public let kind: RelayMessageKind
+    public let payload: Data
+
+    public init<Value: Encodable>(kind: RelayMessageKind, value: Value) throws {
+        self.kind = kind
+        self.payload = try RelayJSON.encode(value)
+    }
+}