Fix audio underflow on stream start by buffering before playback - #232

Problem

The ALSA stream starts immediately when the first audio chunk arrives (queue.qsize() > 0), with only ~25ms of audio buffered. The PortAudio callback drains this single chunk before the next one arrives over the network, causing an immediate underflow.

This triggers a clear/re-anchor cycle that can cascade into repeated underflows:

INFO:sendspin.audio:Stream STARTED: 1 chunks, 0.03 seconds buffered
INFO:sendspin.audio:Sync error 514.3 ms too large; scheduling reanchor
WARNING:sendspin.audio:Audio underflow detected; requesting re-anchor
INFO:sendspin.audio:Cleared audio queue after underflow (deferred from audio thread)

This is especially problematic with USB audio devices that have larger period sizes (2048 frames / ~42ms at 48kHz), where the output latency alone exceeds the buffered audio duration.

Fix

Wait until sufficient audio is buffered before starting the stream, using the existing _MIN_BUFFER_DURATION_US (200ms) and _MIN_CHUNKS_TO_START (16) constants:

# Before (starts with ~25ms buffer):
if not self._stream_started and self._queue.qsize() > 0 and self._stream is not None:

# After (waits for 200ms or 16 chunks):
if not self._stream_started and self._stream is not None and (
    self._queued_duration_us >= self._MIN_BUFFER_DURATION_US
    or self._queue.qsize() >= self._MIN_CHUNKS_TO_START
):

This adds ~200ms of initial latency but eliminates the underflow/re-anchor cascade. After the fix:

INFO:sendspin.audio:Stream STARTED: 8 chunks, 0.20 seconds buffered

Environment

• sendspin 7.0.0
• Raspberry Pi 5 (aarch64)
• USB audio: Wondom GAB8 8-channel amplifiers (period_size=2048, buffer_size=16384)
• PortAudio V19.6.0-devel, MMAP_INTERLEAVED access