First step toward a visionOS target (Option A in the plan: add visionOS as a Supported Destination on the existing Meshtastic target rather than a second app target). This PR handles the source-side prep; the Xcode project wiring is a follow-up that must be done in Xcode's target editor.

What changed?

Wrapped all iOS-only subsystems so the Meshtastic target can compile against the visionOS SDK once the destination is added:

• CarPlay (Meshtastic/CarPlay/*) — #if os(iOS) && canImport(CarPlay)
• SiriKit Intents (Meshtastic/Intents/*, 5 files using legacy INExtension / INSendMessageIntent etc.) — #if os(iOS)
• UIApplicationDelegate (MeshtasticAppDelegate.swift, plus the @UIApplicationDelegateAdaptor declaration and appDelegate.router wire-up in MeshtasticApp.swift) — #if os(iOS)
• Live Activities (Widgets/WidgetsLiveActivity.swift, Widgets/MeshActivityAttributes.swift) — #if os(iOS) (ActivityKit is iOS-only)
• Call sites — two CarPlayIntentDonation.donate… calls in MeshPackets.swift and AccessoryManager+ToRadio.swift guarded so they compile out on non-iOS
• Tests — MeshtasticTests/CarPlayTests.swift gated with #if os(iOS)

Why did it change?

visionOS lacks CarPlay, SiriKit legacy intents, ActivityKit, and UIApplicationDelegate. Without these guards the existing target won't compile for the xrOS SDK. Using #if os(iOS) (which excludes visionOS) keeps a single multi-platform target per Apple's current Xcode 16 guidance — Option B (separate MeshtasticVision target) would duplicate Core Data, AccessoryManager, and routing code.

Follow-up PRs (must be done in Xcode, not safely doable by editing project.pbxproj headlessly):

• PR 2: add Apple Vision to Supported Destinations on Meshtastic + MeshtasticTests, set XROS_DEPLOYMENT_TARGET = 2.0, add a visionOS App Icon set, strip unsupported UIBackgroundModes (bluetooth-central/-peripheral/voip) for the visionOS destination, update README platform matrix.
• PR 3: visionOS UI polish (ornaments, .glassBackgroundEffect()).

WidgetsExtension intentionally stays iOS-only (no Live Activities on visionOS).

How is this tested?

Compile-time only changes; verified by confirming all #if/#endif pairs balance and that every reference to a guarded type (CarPlayIntentDonation, IntentHandler, INSendMessageIntent, etc.) is itself inside a guarded region. Existing iOS / Mac Catalyst builds are unaffected because #if os(iOS) is true for both. Full validation of the visionOS build path happens in PR 2 once the destination is added in Xcode.

Screenshots/Videos (when applicable)

N/A — no UI changes.

Checklist

• My code adheres to the project's coding and style guidelines.
• I have conducted a self-review of my code.
• I have commented my code, particularly in complex areas.
• I have verified whether these changes require an update to existing documentation or if new documentation is needed, and created an issue in the docs repo if applicable.
• I have tested the change to ensure that it works as intended.