[BUG] WebRtcApmModifier: AEC silently does nothing - far-end (reverse) stream is never fed to APM

Nobody has claimed this yet.

### Requirements

- [x] I have searched the existing issues to ensure this bug has not already been reported.
- [x] I have provided a clear and concise description of the bug.
- [x] I have provided detailed steps to reproduce the bug.
- [x] I have included a minimal reproducible example (MRE) if applicable.
- [x] I have provided error messages and stack traces if applicable.
- [x] I have completed the environment information.

### 1. Describe the Bug

## Summary
When using `WebRtcApmModifier` with AEC enabled on a `FullDuplexDevice`, the acoustic echo canceller never receives a far-end reference signal, so `ProcessReverseStream` is effectively never called and echo cancellation does nothing. Noise suppression (NS) and the high-pass filter still work on the near-end capture stream, which makes the audio *sound* processed and hides the fact that AEC is a no-op.

## Root cause
`WebRtcApmModifier` wires its internal far-end handler (`HandleAudioEngineProcessedForAec`, which only acts on `Capability.Playback` frames) to the `FullDuplexDevice`/engine `OnAudioProcessed` event. However, that event is raised **only for the capture device path** — playback/render devices never raise `OnAudioProcessed`. As a result the handler is only ever called with capture (near-end) frames, which it ignores, and it is never called with the playback (far-end) frames it actually needs. The reverse stream therefore stays empty and AEC has no reference signal.

## Suggested fix
The modifier needs to actually receive the render/playback post-mix as the far-end signal. Either:
- raise the processed-audio event on playback/render devices as well (so the existing `Capability.Playback` handler gets fed), or
- inside `WebRtcApmModifier`, explicitly drive `ProcessReverseStream` from the playback device's master mixer instead of relying on the capture-only `OnAudioProcessed` event.

### 2. Steps to Reproduce

1. Create a `FullDuplexDevice` and attach `WebRtcApmModifier` with `aecEnabled: true` (NS/HPF on or off — doesn't matter).
2. Play back a signal on the playback device while capturing on the mic in a shared acoustic path (speakers audible to the mic).
3. Observe that captured audio still contains the full echo of the playback signal — AEC has no effect. Instrumenting `ProcessReverseStream` shows it is never called with real render data.

### 3. Expected Behavior

With `aecEnabled: true`, the APM's reverse stream should be continuously fed with the render/playback signal (what is being sent to the speakers), so AEC3 can build its reference and cancel echo from the capture stream.

### 4. Current Behavior

The far-end handler is registered but never invoked with playback audio, so `ProcessReverseStream` is never driven. AEC produces no cancellation; the caller hears their own echo. Because NS/HPF still run on the near-end, there is no error and the output is not obviously broken — the failure is silent.

### 5. Minimal Reproducible Example (MRE)

```csharp

```

### 6. Error Messages and Stack Trace (if applicable)

```csharp

```

### SoundFlow Version

1.4.0

### .NET Version

.NET 10.0

### Operating System

Windows 11

### Architecture

x64

### Audio Backend Used (if known)

Full-duplex WASAPI device

### Specific Audio Hardware (if relevant)

_No response_

### 8. Affected Components/Modules (if known)

`SoundFlow.Extensions.WebRtc.Apm` 1.4.0

### 9. Impact

_No response_

### 10. Possible Workaround (if known)

I route the playback master-mix into the modifier's private far-end handler ourselves via a small `SoundModifier` attached to `PlaybackDevice.MasterMixer`, invoking the internal `HandleAudioEngineProcessedForAec` with `Capability.Playback` (currently via reflection, since the method is non-public). The buffer is passed through unmodified. With this in place AEC converges and cancels correctly.

### 11. Additional Context

_No response_

Start by tracing WebRtcApmModifier's HandleAudioEngineProcessedForAec registration through FullDuplexDevice.OnAudioProcessed, then inspect the playback device's MasterMixer and ProcessReverseStream path. Done means the APM receives the render/playback signal as its reverse stream and AEC cancels the captured echo while near-end processing remains intact.

Written by the indexing model from the issue text.