gui: Add terminator packet and 200ms voice hold for VAD

Implements proper voice activity detection with:
- 200ms hold period after audio drops below threshold to prevent choppy cutoffs
- Terminator packet (end_bit=true) when speech ends to signal stream completion
- TransmitState enum to track transmission state across frames

This ensures other Mumble clients receive proper end-of-speech signaling
for clean audio termination and correct "talking" indicator behavior.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gui/src/imp/native_audio.rs

@@ -1,4 +1,4 @@
-use crate::effects::{AudioProcessor, AudioProcessorSender};
+use crate::effects::{AudioProcessor, AudioProcessorSender, TransmitState};
 use color_eyre::eyre::{eyre, Error};
 use cpal::traits::{DeviceTrait, HostTrait, StreamTrait as _};
 use futures::io::{AsyncRead, AsyncWrite};
@@ -23,6 +23,31 @@ pub struct AudioSystem {
 const SAMPLE_RATE: u32 = 48_000;
 const PACKET_SAMPLES: u32 = 960;
 
+fn encode_and_send(
+    state: TransmitState,
+    output_buffer: &mut Vec<f32>,
+    encoder: &mut opus::Encoder,
+    each: &mut impl FnMut(Vec<u8>, bool),
+) {
+    let (is_terminator, should_encode) = match state {
+        TransmitState::Silent => return,
+        TransmitState::Transmitting => (false, output_buffer.len() >= PACKET_SAMPLES as usize),
+        TransmitState::Terminator => {
+            output_buffer.resize(PACKET_SAMPLES as usize, 0.0);
+            (true, true)
+        }
+    };
+
+    if should_encode {
+        let remainder = output_buffer.split_off(PACKET_SAMPLES as usize);
+        let frame = replace(output_buffer, remainder);
+        match encoder.encode_vec_float(&frame, frame.len() * 2) {
+            Ok(encoded) => each(encoded, is_terminator),
+            Err(e) => error!("error encoding {} samples: {e:?}", frame.len()),
+        }
+    }
+}
+
 type Buffer = Arc<Mutex<dasp_ring_buffer::Bounded<Vec<i16>>>>;
 
 impl AudioSystem {
@@ -79,7 +104,7 @@ impl AudioSystem {
 
     pub fn start_recording(
         &mut self,
-        mut each: impl FnMut(Vec<u8>) + Send + 'static,
+        mut each: impl FnMut(Vec<u8>, bool) + Send + 'static,
     ) -> Result<(), Error> {
         let config = self.choose_config(self.input.supported_input_configs()?)?;
         info!(
@@ -97,20 +122,8 @@ impl AudioSystem {
             if let Some(new_processor) = processors.take() {
                 current_processor = new_processor;
             }
-            current_processor.process(frame, config.channels as usize, &mut output_buffer);
-            if output_buffer.len() < PACKET_SAMPLES as usize {
-                return;
-            }
-            let remainder = output_buffer.split_off(PACKET_SAMPLES as usize);
-            let frame = replace(&mut output_buffer, remainder);
-            match encoder.encode_vec_float(&frame, frame.len() * 2) {
-                Ok(buf) => {
-                    each(buf);
-                }
-                Err(e) => {
-                    error!("error encoding {} samples: {e:?}", frame.len());
-                }
-            }
+            let state = current_processor.process(frame, config.channels as usize, &mut output_buffer);
+            encode_and_send(state, &mut output_buffer, &mut encoder, &mut each);
         };
 
         match self