Watch: snapshot tests render the complication and multi-select screenshots

ImageRenderer draws the real views with demo data, since the watch simulator
is not drivable here. README covers the widget target and how to add it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

watch/WristCode/Views/ThreadView.swift

@@ -18,24 +18,52 @@ struct ThreadView: View {
     @State private var selections: [String: Set<String>] = [:]
     @State private var scrolledToLatest = false
 
+    /// Snapshots render the column without its ScrollView, which ImageRenderer leaves blank.
+    private let scrolls: Bool
+
+    /// `detail` and `selections` start the screen with a thread loaded and options ticked, for
+    /// snapshots and previews.
+    init(
+        hostId: String, threadId: String,
+        detail: ThreadDetail? = nil, selections: [String: Set<String>] = [:], scrolls: Bool = true
+    ) {
+        self.hostId = hostId
+        self.threadId = threadId
+        self.scrolls = scrolls
+        _detail = State(initialValue: detail)
+        _selections = State(initialValue: selections)
+    }
+
     var body: some View {
+        if scrolls {
+            screen
+        } else {
+            column
+        }
+    }
+
+    private var column: some View {
+        VStack(alignment: .leading, spacing: 8) {
+            if let detail {
+                content(detail)
+            } else if error == nil {
+                Text("Loading")
+                    .font(.system(size: 15, weight: .semibold))
+                    .foregroundStyle(Theme.secondaryText)
+            }
+            if let error {
+                Text(error)
+                    .font(.system(size: 12))
+                    .foregroundStyle(Theme.secondaryText)
+            }
+        }
+        .padding(.horizontal, 4)
+    }
+
+    private var screen: some View {
         ScrollViewReader { proxy in
             ScrollView {
-                VStack(alignment: .leading, spacing: 8) {
-                    if let detail {
-                        content(detail)
-                    } else if error == nil {
-                        Text("Loading")
-                            .font(.system(size: 15, weight: .semibold))
-                            .foregroundStyle(Theme.secondaryText)
-                    }
-                    if let error {
-                        Text(error)
-                            .font(.system(size: 12))
-                            .foregroundStyle(Theme.secondaryText)
-                    }
-                }
-                .padding(.horizontal, 4)
+                column
             }
             .onChange(of: detail?.messages.last?.id) {
                 guard !scrolledToLatest, detail != nil else { return }
@@ -360,7 +388,7 @@ private struct QuickReplies: View {
 
 #Preview("Multi-select") {
     NavigationStack {
-        ThreadView(hostId: "mac-mini", threadId: "t-release")
+        ThreadView(hostId: "mac-mini", threadId: "t-release", selections: ["checks": ["unit", "lint"]])
     }
     .environment(AppModel.preview())
 }