Mark a loop watching the Mailroom with an envelope glyph

graphcode/Sources/Features/App/AppSidebarView.swift

@@ -282,6 +282,12 @@ struct AppSidebarView: View {
         .frame(width: 3, height: 14)
       Text(node.title).font(.system(size: 12.5)).lineLimit(1)
       Spacer(minLength: 4)
+      if let watching = MailroomWatchPresentation.tooltip(for: node) {
+        Image(systemName: MailroomWatchPresentation.symbolName)
+          .font(.system(size: 10))
+          .foregroundStyle(.white.opacity(0.45))
+          .help(watching)
+      }
       Text(LoopCardPresentation.duration(sidebarNow.timeIntervalSince(node.createdAt)))
         .font(.system(size: 10.5, design: .monospaced))
         .foregroundStyle(.white.opacity(0.5))

graphcode/Sources/Features/Canvas/LoopCardView.swift

@@ -241,6 +241,12 @@ struct LoopCardView: View {
       if isRemote {
         Image(systemName: "network").font(.system(size: 9)).foregroundStyle(.white.opacity(0.4))
       }
+      if let watching = MailroomWatchPresentation.tooltip(for: node) {
+        Image(systemName: MailroomWatchPresentation.symbolName)
+          .font(.system(size: 9))
+          .foregroundStyle(.white.opacity(0.4))
+          .help(watching)
+      }
       switch entryRole {
       case .entry: EntryChip()
       case .cycleOnly: CycleChip()

graphcode/Sources/Features/Canvas/MailroomWatchPresentation.swift

@@ -0,0 +1,18 @@
+import GraphcodeKit
+import MailroomKit
+
+/// The envelope that marks a loop watching its project's Mailroom, and its tooltip.
+///
+/// Hidden on a resolved loop: `mailroomWatch` outlives the session that set it, but a
+/// loop that has finished for good cannot be rung.
+enum MailroomWatchPresentation {
+  static let symbolName = "envelope"
+
+  static func showsGlyph(for node: LoopNode) -> Bool { tooltip(for: node) != nil }
+
+  static func tooltip(for node: LoopNode) -> String? {
+    guard let watch = node.mailroomWatch, !node.isResolved else { return nil }
+    guard let topic = watch.topic else { return "Watching the Mailroom" }
+    return "Watching the Mailroom · topic \(topic)"
+  }
+}

graphcode/Tests/MailroomWatchPresentationTests.swift

@@ -0,0 +1,46 @@
+import Foundation
+import GraphcodeKit
+import MailroomKit
+import Testing
+
+@testable import graphcode
+
+@Suite
+struct MailroomWatchPresentationTests {
+  private func node(watch: MailroomWatch?, state: LoopState = .running) -> LoopNode {
+    LoopNode(title: "Triage", loopType: .goalBased, mailroomWatch: watch, state: state)
+  }
+
+  @Test
+  func aLoopWithoutAWatchShowsNothing() {
+    let plain = node(watch: nil)
+
+    #expect(!MailroomWatchPresentation.showsGlyph(for: plain))
+    #expect(MailroomWatchPresentation.tooltip(for: plain) == nil)
+  }
+
+  @Test
+  func aWatchOnEveryPostSaysSoWithoutATopic() {
+    let hearingAll = node(watch: MailroomWatch())
+
+    #expect(MailroomWatchPresentation.showsGlyph(for: hearingAll))
+    #expect(MailroomWatchPresentation.tooltip(for: hearingAll) == "Watching the Mailroom")
+  }
+
+  @Test
+  func aWatchOnOneTopicNamesIt() {
+    let narrowed = node(watch: MailroomWatch(topic: "design"))
+
+    #expect(MailroomWatchPresentation.showsGlyph(for: narrowed))
+    #expect(
+      MailroomWatchPresentation.tooltip(for: narrowed) == "Watching the Mailroom · topic design")
+  }
+
+  @Test(arguments: [LoopState.succeeded, .failed, .stalled, .stopped])
+  func aResolvedLoopHidesItsWatch(state: LoopState) {
+    let finished = node(watch: MailroomWatch(topic: "design"), state: state)
+
+    #expect(!MailroomWatchPresentation.showsGlyph(for: finished))
+    #expect(MailroomWatchPresentation.tooltip(for: finished) == nil)
+  }
+}