feat: WatchDevices — MIDI device hot-plug detection

[leandrodaf]

Summary

Adds WatchDevices(ctx context.Context) (<-chan DeviceEvent, error) to the ClientMIDI interface, enabling consumers to react in real time when a MIDI device is connected or disconnected.

New API

// DeviceEventType describes what happened to a MIDI device.
type DeviceEventType int

const (
    DeviceAdded   DeviceEventType = iota
    DeviceRemoved
)

// DeviceEvent is emitted by WatchDevices when the set of MIDI devices changes.
type DeviceEvent struct {
    Type   DeviceEventType
    Device DeviceInfo
}

// WatchDevices returns a channel that emits DeviceEvent values on hot-plug.
// The channel is closed when ctx is cancelled.
WatchDevices(ctx context.Context) (<-chan DeviceEvent, error)

Per-platform implementation

Platform	Mechanism	Latency
macOS	Native CoreMIDI MIDINotifyProc callback via CGo (cgo.Handle pattern)	Instant
Linux	Polling ListDevices() every 2 s	≤ 2 s
Windows	Polling midiInGetNumDevs every 2 s	≤ 2 s
Stubs / no-CGo	Channel closed on ctx cancel, no events	—

Changes

• sdk/contracts/midi.go — DeviceEventType, DeviceEvent, WatchDevices added to ClientMIDI
• sdk/contracts/mock.go — WatchDevicesFunc / WatchDevicesCalls added to MockMIDIClient
• internal/coremidi/client.go — CGo export goMidiNotify, static C bridge notifyBridgeFn, cgo.Handle lifecycle, Client.NotifyCh, Dispose()
• internal/midi/mididarwin/client_darwin.go — WatchDevices using CoreMIDI notify channel + diffDevices helper; Stop() calls Dispose()
• internal/midi/midilinux/client_linux_cgo.go — WatchDevices with 2 s polling + diffDevices
• internal/midi/midiwindows/client_windows.go — WatchDevices with 2 s polling + diffDevices
• All stubs / dummy clients updated to satisfy the interface

Tests

• mididarwin and midiwindows dummy clients: 100% coverage
• sdk/contracts mock: WatchDevices func, error, cancel, call counter
• midilinux CGo client: channel returned, cancel, diffDevices add/remove/unchanged (via export_test.go shim)

Notes

• cgo.Handle.Delete() is called in ClientMid.Stop() (darwin) via client.Dispose() — no handle leak
• Device identity comparison uses Name + Manufacturer; adequate for single-device use case
• go mod tidy does not accept -tags; tidy was run without build tags

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com