HydraIssues

## Goal
Enable microphone input for `enable_microphone: true` experiences (e.g. mercator-talks) on hydraheadipad, matching the voice path already working on hydraheadflatscreen.

## Current state

The infrastructure is largely in place but the relay is never activated:

- `Sources/HydraHeadiPad/Services/MicrophoneRelay.swift` — AVAudioEngine mic capture → Opus → RTP/UDP to bodyHost:47995. Correct structure, correct protocol (PT=111, 12-byte RTP header, 48 kHz mono, 960 samples/frame matching hydraheadflatscreen).
- `Sources/HydraHeadiPad/Bridging/HydraOpus.h/.c` — C wrapper around opus_encode_float, singleton encoder.
- `Sources/HydraHeadiPad/Models/HeadConfig.swift:63` — `enableMicrophone: Bool?` decoded from catalog JSON `enable_microphone` field.
- `Sources/HydraHeadiPad/Views/ExperienceGridView.swift:86` — mic icon shown when `enableMicrophone == true`.
- `Sources/HydraHeadiPad/Info.plist:32` — `NSMicrophoneUsageDescription` already present.

**MicrophoneRelay is never instantiated or called anywhere.** AppState does not reference it.

## What needs to be done

### 1. Wire MicrophoneRelay into AppState

In `AppState.swift`:
- Add a `private let micRelay = MicrophoneRelay()` property.
- In `startStream(assignment:)`: after the StreamingView is presented, check `assignment.experience.enableMicrophone == true` and call `micRelay.start(bodyHost: host)`.
- In the stream stop/error handlers (or wherever state returns to `.selfService` or `.error`): call `micRelay.stop()`.

The relay is non-fatal — if it fails (permission denied, AVAudioEngine error) the stream continues without mic input.

### 2. Fix AVAudioSession category

`MicrophoneRelay.startAsync` (line 53) sets:
```swift
try AVAudioSession.sharedInstance().setCategory(.playAndRecord, options: .defaultToSpeaker)
```

Missing `.mixWithOthers`. Without it, switching to `.playAndRecord` will disrupt SDL's audio output for the Moonlight stream (SDL sets `.playback` via CoreAudio when `SDL_OpenAudioDevice` runs). The category must be:
```swift
.setCategory(.playAndRecord, options: [.defaultToSpeaker, .mixWithOthers])
```

### 3. Tune Opus encoder (HydraOpus.c)

Currently uses `OPUS_APPLICATION_AUDIO` at 96 kbps. hydraheadflatscreen uses `OPUS_APPLICATION_VOIP` at 64 kbps — VOIP mode is optimised for speech (lower algorithmic delay, better DTX, comfort noise). Change HydraOpus.c to match:
```c
_encoder = opus_encoder_create(48000, 1, OPUS_APPLICATION_VOIP, &error);
opus_encoder_ctl(_encoder, OPUS_SET_BITRATE(64000));
```

### 4. Verify via testbook step 9

Testbook step 9 (Mercator Talks) covers mic permission prompt and voice relay. Run it end-to-end after the AppState wiring is in place.

## Reference

- hydraheadflatscreen mic relay: `pkg/client/micrelay.go`, `micrelay_darwin.go` — reference for RTP format, Opus params, relay lifecycle.
- hydravoice on body: receives UDP RTP at port 47995, decodes Opus, writes to VB-Cable CABLE Input. Unreal reads from CABLE Output.
- enable_microphone flag must be set on the experience in hydraexperiencelibrary.