fix(darwin): create MIDINetworkSession lazily, not at plugin init

[aleksei-svezhevskii]

Problem

On iOS 14+, touching MIDINetworkSession.default() in setup() makes the system show the "Allow [app] to find devices on local networks" permission prompt at app startup — even though the network session is disabled by default (isEnabled == false) and the app may only use Bluetooth/USB/virtual MIDI.

Users see a confusing network-permission dialog before the first screen, with no way for the app to avoid it (the standard NSLocalNetworkUsageDescription string does not remove the prompt, it only changes the body text).

Fix

Stop creating the session at plugin init. Create it lazily inside setNetworkSessionEnabled(true) — which is already the documented, explicit opt-in for network (RTP) MIDI (see the iOS-specific API added in 0.4.15).

func setNetworkSessionEnabled(enabled: Bool) throws {
#if os(iOS)
    if enabled && session == nil {
        session = MIDINetworkSession.default()
        session?.connectionPolicy = MIDINetworkConnectionPolicy.anyone
    }
    session?.isEnabled = enabled
#endif
}

Impact

• Bluetooth / USB / virtual MIDI: unaffected — they go through CoreMIDI without a network session.
• Network MIDI via setNetworkSessionEnabled(true): unchanged — the session is created at the same call, identical behavior afterwards.
• isNetworkSessionEnabled() still reports false before any session exists, matching the disabled-by-default semantics.

Testing

Verified on the iOS simulator with an app using this plugin:

• the system Local Network prompt no longer appears at launch;
• USB/virtual device enumeration is unchanged;
• setNetworkSessionEnabled(true) still creates and enables the network session.

[mortenboye]

Thank you for your contribution, this will go into the next reelase.