feat(rtmp): FrameLifecycleListener for pooled-copy buffer management

[pird32]

Problem

At 30 fps video + 50 fps audio, the RTMP send path requires a defensive ByteBuffer.clone() before each sendMediaFrame() call because MediaCodec output buffers are reclaimed after the callback returns. This creates ~80 heap allocations per second, which causes GC pressure and monotonic heap growth under sustained load.

Callers currently have no way to know when the sender has finished reading the buffer, so they cannot safely implement a buffer pool — the recycled buffer might still be referenced by a queued frame.

Solution

This PR (which builds on the shared contract from PR #N) exposes a per-sender lifecycle hook:

rtmpSender.frameLifecycleListener = FrameLifecycleListener { frame ->
    // called from the sender's IO thread AFTER network write
    myPool.release(frame.data)
}

notifyFrameConsumed(frame) is called from RtmpSender.onRun() after each frame's network write completes. The sender does not access frame.data after this point.

Also emits ConnectChecker.onTransportEvent(NetworkSendError) from the error path alongside the existing onConnectionFailed() call, for structured error handling.

Thread contract

onFrameConsumed is called on the sender's Dispatchers.IO coroutine. Implementations must not block.

Backward compatibility

frameLifecycleListener defaults to null. Existing senders behave identically with no listener registered. Callers that currently use clone() can continue to do so — this is purely additive.

Example: zero-allocation RTMP send

val pool = ByteBufferPool(capacity = 128)
rtmpSender.frameLifecycleListener = FrameLifecycleListener { frame ->
    pool.release(frame.data)
}

// In MediaCodec callback:
val buf = pool.acquire(videoBuffer) ?: return // pool exhausted = queue full, drop
rtmpSender.sendMediaFrame(MediaFrame(buf, info.toMediaFrameInfo(), MediaFrame.Type.VIDEO))

[pird32]

Closing this PR to keep only one consolidated PR for easier review/testing, as requested. All changes are included in #2072.