fix(audio): Remove clipping from WavDumpBackend, add diagnostics

Fixed design flaw where WavDumpBackend was clamping samples to [-1.0, 1.0]
before writing to file. This prevented detection of audio problems.

Changes:
- Removed sample clamping (lines 57-60 in old code)
- WAV dump now records audio "as is" (matches MiniaudioBackend behavior)
- Added clipped_samples_ counter to track diagnostic metric
- Added get_clipped_samples() method for programmatic access
- Report clipping statistics in shutdown():
  - "✓ No clipping detected" when clean
  - "WARNING: N samples clipped (X% of total)" when clipping occurs
  - Suggests reducing volume to fix

Why this matters:
- MiniaudioBackend does NOT clip samples (passes directly to miniaudio)
- WavDumpBackend should match this behavior
- Clipping in WAV files helps identify audio distortion problems
- Developers can compare WAV output to expected values
- Diagnostic metric helps tune audio levels

Testing:
- Added test_clipping_detection() test case
- Verifies clipping counter works correctly (200 clipped / 1000 samples)
- Existing tests show "✓ No clipping detected" for normal audio
- All 27 tests pass

Example output:
  WAV file written: test.wav (2.02 seconds, 128986 samples)
  ✓ No clipping detected

  WAV file written: loud.wav (10.5 seconds, 336000 samples)
  WARNING: 4521 samples clipped (1.35% of total)
  This indicates audio distortion - consider reducing volume

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

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