chore: remove dead test code and fix stale comment

tests/test_isr.py:
- Remove unused imports: io, SimpleNamespace, call
- Remove test_mp3_chunks_written_to_file — it mocked connect_stream but
  never called record(), making all the mock scaffolding dead; the actual
  assertion (bytes written to a file) is already covered by
  TestAudioFileWriter and TestGenerateFilename

isr.py:
- Update AudioDevice.backend comment: only the ALSA backend exists now

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_isr.py

@@ -4,15 +4,13 @@ Tests for isr.py
 Run with:  pytest tests/
 """
 
-import io
 import logging
 import struct
 import wave
 from datetime import datetime
 from pathlib import Path
-from types import SimpleNamespace
 from typing import List
-from unittest.mock import MagicMock, patch, call
+from unittest.mock import MagicMock, patch
 
 import pytest
 
@@ -571,50 +569,6 @@ class TestStreamRecorderRecord:
             )
         return r
 
-    def test_mp3_chunks_written_to_file(self, tmp_path):
-        chunks = [b"A" * 512, b"B" * 512, b"C" * 512]
-        rec = self._recorder(fmt="mp3")
-        rec.output_dir = str(tmp_path)
-
-        mock_resp = MagicMock()
-        mock_resp.headers = {"Content-Type": "audio/mpeg"}
-        mock_resp.iter_content.return_value = iter(chunks)
-
-        def _stop_after_connect(*args, **kwargs):
-            rec.running = True
-            return mock_resp
-
-        with patch.object(rec, "connect_stream", side_effect=[mock_resp, None]):
-            # connect_stream returns the mocked response on the first call;
-            # we stop the loop via a side-effectful iter_content
-            original_iter = mock_resp.iter_content.return_value
-
-            def _chunks_then_stop(chunk_size=8192):
-                for c in chunks:
-                    yield c
-                rec.running = False  # stop the outer while loop
-
-            mock_resp.iter_content.side_effect = _chunks_then_stop
-            mock_resp.headers = {"Content-Type": "audio/mpeg"}
-            rec.detected_format = "mp3"
-            rec.running = True
-            rec.header_capture_complete = True
-            rec.stream_headers = None
-
-            # Open a file manually so we can verify writes
-            filename = rec.generate_filename("mp3")
-            rec.current_file = open(filename, "wb")
-            rec.current_filename = filename
-
-            # Simulate one inner loop iteration
-            for chunk in _chunks_then_stop():
-                if chunk:
-                    rec.current_file.write(chunk)
-            rec.close_current_file()
-
-        written = Path(filename).read_bytes()
-        assert written == b"A" * 512 + b"B" * 512 + b"C" * 512
-
     def test_connection_failure_retries(self, tmp_path):
         rec = self._recorder()
         rec.output_dir = str(tmp_path)