Fix ffmpeg backend misidentifying metadata as audio stream info

[gaoflow]

What

When a file's metadata contains the substring "audio:" (e.g. a
description tag whose value reads "audio: broken"), _get_info() in
the ffmpeg backend stops reading stderr on that metadata line instead of
continuing until the real stream descriptor (Stream #N:M: Audio: …).
This silently produces channels = 0, samplerate = 0, and duration = 0
for the decoded file, which causes playback failures downstream.

Why it happens

# ffdec.py – before fix
elif 'audio:' in line:     # matches metadata lines too
    out_parts.append(line)
    self._parse_info(''.join(out_parts))
    break

ffmpeg prints file metadata before the stream table, so any tag whose
value contains "audio:" hits this branch first:

Input #0, mp3, from 'out.mp3':
  Metadata:
    description     : audio: broken   ← wrong line matched here
    ...
  Duration: 00:00:02.04, ...          ← never reached
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, ...

Fix

Require 'stream #' to also be present on the matched line. ffmpeg
always prefixes stream entries with Stream #N:M:, so the real audio
line always contains both substrings, while metadata value lines never
contain 'stream #'.

elif 'stream #' in line and 'audio:' in line:

Closes #119.

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This pull request was prepared with the assistance of AI, under my direction and review.