iOS: registry-driven auto-attach (sign in → connected)

A signed-in phone whose team has one reachable Mac now connects on the
cold-start path with no QR scan or manual host entry. The attach ticket is
route-discovery only; the Mac authorizes the mint purely on matching Stack
account, so a registry route + Stack token is sufficient (no prior pairing).

- MobileAutoAttachTargetSelector: pure target picker (online-preferred, else
  single most-recently-seen with a reachable route; ambiguity → nil → manual).
- MobileAttachRoutePriority: shared route-priority helper (single source of
  truth for reconnect, switch, device-tree tap, auto-attach).
- MobileAutoAttachFlag: mobileAutoAttach flag, DEBUG on / Release off, override
  via cmux.mobile.autoAttach.enabled.
- attemptAutoAttachIfEligible chains from the no-stored-mac reconnect branch;
  bounded, cancellable, one attempt per generation; reuses connectToRegistryInstance
  (Stack-authenticated mint + paired-mac persist) so the next launch takes the
  normal reconnect path.
- Presence seam optional (MobileAutoAttachPresenceProviding); degrades to
  recency until presence (#5792) lands.

Tests: pure selector (online>recency, no-candidate, ambiguity) + composite
behavior (connects+persists, fall-through, one-attempt-per-generation, flag off,
already-connected). swift test green for CmuxMobileShellModel and CmuxMobileShell;
iOS simulator arm64 build succeeds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: lawrencecchen

Packages/CmuxMobileShell/Sources/CmuxMobileShell/MobileAutoAttachPresenceProviding.swift

@@ -0,0 +1,16 @@
+public import Foundation
+
+/// An online/offline signal for registry-driven auto-attach target selection.
+///
+/// Optional seam: when no presence provider is wired (the presence service is
+/// not merged yet, #5792), auto-attach degrades to most-recently-seen selection,
+/// which is correct for the dominant single-Mac team. Once the presence client
+/// lands, an adapter conforming to this protocol lets auto-attach prefer the live
+/// Mac and treat multiple live Macs as ambiguous (fall through to manual pair)
+/// rather than guessing on recency.
+public protocol MobileAutoAttachPresenceProviding: Sendable {
+    /// The device ids the presence service currently reports as online, scoped to
+    /// the signed-in user's team. Best-effort: a `nil` snapshot means "no
+    /// presence signal right now," so selection falls back to recency.
+    func onlineDeviceIDs() async -> Set<String>?
+}