feat(chat): per-turn stopwatch on assistant bubbles (Phase 2.2)

Wall-clock duration of each agent turn renders as a compact pill in
the message metadata footer (Mac) / below the bubble (iOS). Mirrors
the per-turn stopwatch Hermes v2026.4.23's TUI rewrite ships.

ScarfCore RichChatViewModel:
- currentTurnStart: Date? captured in addUserMessage when entering a
  fresh turn (skipped for /steer-style mid-run sends so the duration
  reflects the FULL turn).
- turnDurations: [Int: TimeInterval] keyed by finalised assistant
  message id; populated in finalizeStreamingMessage and cleared on
  reset().
- formatTurnDuration(_:) static — "0.8s" / "4.2s" / "1m 12s".

Mac:
- RichMessageBubble gains turnDuration: TimeInterval?; renders via
  formatTurnDuration in the existing metadata footer.
- RichChatMessageList + MessageGroupView thread the durations dict
  through; RichChatView wires richChat.turnDurations.

iOS:
- MessageBubble gains turnDuration parameter; renders below the
  bubble for assistant messages only.
- ChatView's ForEach passes controller.vm.turnDuration(forMessageId:).

Verified: Mac + iOS builds clean. Resumed sessions (loaded from
state.db) show no pill — turnDurations only populates for live ACP
turns, which is the correct behaviour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/Scarf iOS/Chat/ChatView.swift

@@ -165,8 +165,11 @@ struct ChatView: View {
                     }
                 }
                 ForEach(controller.vm.messages) { msg in
-                    MessageBubble(message: msg)
-                        .id(msg.id)
+                    MessageBubble(
+                        message: msg,
+                        turnDuration: controller.vm.turnDuration(forMessageId: msg.id)
+                    )
+                    .id(msg.id)
                 }
                 if controller.vm.isGenerating {
                     HStack {
@@ -933,6 +936,11 @@ private struct PermissionWrapper: Identifiable {
 
 private struct MessageBubble: View {
     let message: HermesMessage
+    /// Wall-clock duration of the agent turn this assistant message
+    /// belongs to (v2.5). Renders as a small `4.2s` pill below the
+    /// bubble when present. Nil for user / streaming / pre-v2.5
+    /// resumed messages.
+    var turnDuration: TimeInterval? = nil
 
     var body: some View {
         if message.isToolResult {
@@ -963,6 +971,13 @@ private struct MessageBubble: View {
                             }
                         }
                     }
+                    // Per-turn stopwatch — assistant only, when the
+                    // turn duration was captured (live ACP turns).
+                    if !message.isUser, let seconds = turnDuration {
+                        Text(RichChatViewModel.formatTurnDuration(seconds))
+                            .font(.caption2)
+                            .foregroundStyle(.tertiary)
+                    }
                 }
                 if !message.isUser { Spacer(minLength: 40) }
             }