iOS: Siri and Shortcuts, and a widget (#427, #428)

Siri: App Intents with phrases that need no setup: Resume, Pause,
Skip forward, Skip back, Set a sleep timer, How many unread. The
playback ones are AudioPlaybackIntents, run in the app and allowed to
start audio from the background. Playback is now one shared instance
(Playback.shared) so Siri and the widget reach it with no page loaded.
It remembers the last episode and its place (LastPlayed), so Resume
works with the app closed; the page, opened afterwards, does not know
about an episode started that way.

Widget: small and medium on the home screen, circular and rectangular
on the lock screen. It shows the unread count and the last episode,
with a Resume or Pause button (PlayPauseIntent, shared with Siri). It
draws what the app writes to the App Group group.net.sdf1.ipx. The app
asks for a redraw on play, pause, a new episode or a new count, not on
every position, since iOS rations redraws. Shared/ builds into both
targets, with WIDGET set in the widget's, where the player does not
exist.

ipx's push now carries content-available, which wakes the closed app
for a moment to give the widget the count the icon just got (#433).

Tested: unit tests for the snapshot and the remembered place, 19 in
all; in the simulator, playing an episode wrote the snapshot to the
App Group. Not testable there: Siri, and the widget drawn.

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

ios/Sources/LastPlayed.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+/// The episode the player last had and where it got to, kept across launches, so "Resume iPX"
+/// (#427) and the widget's button carry on with it when the app was closed and no page is loaded
+/// to say what to play.
+enum LastPlayed {
+    private static let itemKey = "ipx.last.item"
+    private static let positionKey = "ipx.last.position"
+    static var defaults = UserDefaults.standard
+
+    static var item: Playback.Item? {
+        get { defaults.data(forKey: itemKey).flatMap { try? JSONDecoder().decode(Playback.Item.self, from: $0) } }
+        set {
+            // A different episode starts from where the page put it, not the last one's place.
+            if newValue != item { defaults.removeObject(forKey: positionKey) }
+            defaults.set(newValue.flatMap { try? JSONEncoder().encode($0) }, forKey: itemKey)
+        }
+    }
+
+    static var position: Double? {
+        get { defaults.object(forKey: positionKey) as? Double }
+        set { defaults.set(newValue, forKey: positionKey) }
+    }
+}