Reactive CoreSimulator device state (replace simctl list polling)

[EvanBacon]

Why

Device enumeration went through xcrun simctl list devices, spawned per call across the CLI and middleware (~50ms each), and the web grid polled /grid/api on a fixed 750ms–3s interval. simctl is a thin CLI over CoreSimulator.framework, which already holds a live, XPC-subscribed view of the device set from the com.apple.CoreSimulator.CoreSimulatorService daemon. We can read and subscribe to that directly, in process, and push changes to the UI instead of polling.

How

• Added SimDeviceMonitor (Sources/SimNative), a process-global subscriber that drives CoreSimulator through the Objective-C runtime (dlopen + @objc protocol shims, no linking). It holds the default device set, registers set-level handlers (add/remove) plus per-device handlers (state changes), and keeps a live snapshot. Exposed over N-API as a synchronous listDevices() and a SimWatch push class.
• TS device.ts now serves the simctl list devices -j shape from that reactive snapshot via listDevicesByRuntime / tryListDevicesByRuntime, with a one-shot simctl fallback when the native addon is unavailable. The null-returning variant preserves the lookup-failed vs empty distinction so stale-helper pruning stays safe. findBootedDevice, resolveDevice, and the per-request reads in index.ts / middleware.ts route through it; the old time-based booted-set caches are gone.
• Live device grid: a new /grid/api/events SSE endpoint pushes the same payload as GET /grid/api whenever the device set changes (reactive subscriber) or a helper starts/stops (state-file watch). The client useGridDevices hook subscribes over the existing exec websocket instead of polling. Changes made outside serve-sim (Simulator.app, simctl, Xcode) now reflect in the UI immediately.
• Factored the duplicated SSE scaffold shared by /api/events and /grid/api/events into one streamDeviceStateSse helper.

Test Plan

• tsc --noEmit and oxlint clean; native addon builds (build.ts).
• listDevices() output matches simctl list devices -j exactly (52/52 devices); first call ~600ms (subscribe), cached reads ~0.3ms.
• Ran the built CLI server and opened the /grid/api/events stream while driving a device through simctl: observed live pushes Shutdown -> Booting -> Booted -> Shutting Down -> Shutdown with no polling, before and after the SSE refactor.

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