Claude: add support for multichannel interfaces on iOS/iPadOS

[rdica]

Short description of changes

Adds support for external multichannel interfaces and provides ability to choose inputs on iOS/iPadOS devices.

CHANGELOG: Added support for external multichannel interfaces and ability to choose inputs on iOS/iPadOS devices.

Context: Fixes an issue?

Fixes: 3735

Does this change need documentation? What needs to be documented and how?

I don't know.

Status of this Pull Request

What is missing until this pull request can be merged?

This absolutely needs testing as I have no means to do so.

I don't really expect this to work but hopefully it provides a starting point.

Claude summary:
What was missing: src/sound/coreaudio-ios/sound.mm hardcoded a 2-channel (stereo) input format on the RemoteIO audio unit and never queried AVAudioSession for a device's actual channel count, so any external multichannel interface (USB/Lightning audio interface) was silently truncated to its first two channels.

Changes (src/sound/coreaudio-ios/sound.{h,mm}):

• Negotiate channel count via AVAudioSession.maximumInputNumberOfChannels / setPreferredInputNumberOfChannels: / inputNumberOfChannels whenever a device is selected (SwitchDevice).
• Split the single audio format into separate input (now iNumInChan-wide) and output (still stereo, since iOS playback is always 2-channel) AudioStreamBasicDescriptions applied to the RemoteIO unit.
• GetAvailableInOutDevices() now lists every AVAudioSessionPortDescription in availableInputs (built-in mic, headset, external interfaces) instead of just "default" vs "built-in mic".
• Added UpdateInputChannelInfo() to populate per-channel names (from AVAudioSessionChannelDescription) and clamp the current selection when the device changes.
• processBufferList() now indexes the interleaved N-channel buffer using the user-selected left/right channel instead of assuming channel 0/1.
• Implemented GetNumInputChannels, GetInputChannelName, SetLeft/RightInputChannel, GetLeft/RightInputChannel overrides — these plug straight into the existing generic channel-selection UI in clientsettingsdlg.cpp, which is already gated on __APPLE__ (true for iOS) and just was never exercised because the backend always reported 2 channels.

No GUI changes were needed — the settings dialog's channel-selection frame already supports any platform reporting more than 2 channels.

Caveat: I don't have macOS/Xcode in this environment to actually build and run on a device, so this hasn't been compiled or tested against real hardware — please build it in Xcode and verify with an actual multichannel interface (e.g. plugged in via USB-C/Lightning) before merging.

END Summary

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