Decide complication pushes from the next appointment's identity

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Sources/CalendarrCore/Transport/WatchPushPolicy.swift

@@ -0,0 +1,35 @@
+import Foundation
+
+/// Identifies the appointment a complication is currently showing.
+///
+/// The start date is part of the identity on purpose: a moved event keeps its
+/// id, and the complication is wrong until it is told.
+public struct NextEventIdentity: Equatable, Sendable, Codable {
+    public let id: String
+    public let start: Date
+
+    public init(id: String, start: Date) {
+        self.id = id
+        self.start = start
+    }
+}
+
+public enum WatchPushPolicy {
+    /// The first event that has not ended yet — which is what every "next
+    /// appointment" complication renders, including one that is running now.
+    public static func nextEventIdentity(in snapshot: CalendarrSnapshot,
+                                         at now: Date) -> NextEventIdentity? {
+        snapshot.events
+            .filter { $0.end > now }
+            .min { $0.start < $1.start }
+            .map { NextEventIdentity(id: $0.id, start: $0.start) }
+    }
+
+    /// Whether this change is worth one of the day's limited complication
+    /// pushes. Only a changed next appointment is — everything else reaches the
+    /// watch on the next application-context delivery.
+    public static func shouldPushComplicationUpdate(previous: NextEventIdentity?,
+                                                    current: NextEventIdentity?) -> Bool {
+        previous != current
+    }
+}