Expand DeepRhythm test suite

[bleugreen]

Summary

Expands the test suite from 7 tests in a single file to 59 tests across 4 files organized into tiers by dependency weight.

Test Structure

File	Tier	Tests	Dependencies
tests/test_utils.py	1	18	Pure unit tests — BPM conversion, split_audio, get_device, error classes
tests/test_audio_proc.py	2	19	Audio processing — bandfilter, onset strength, HCQM (CPU tensors, no model weights)
tests/test_model.py	1+3	8	Model architecture (no weights) + 2 slow predictor tests
tests/test_predictor.py	3+4	14	Integration & edge cases — prediction modes, click tracks, silence, boundary conditions

Key Features

• @pytest.mark.slow marker for tests requiring model weights (16 tests)
• Session-scoped predictor fixture — loads model once across all slow tests
• Fast test path: pytest -m "not slow" runs 43 tests in ~2s with no network/weight download
• Parametrized click track tests at 90/120/140 BPM with ±10% tolerance
• Shared helpers in conftest.py: make_click_track, make_sine_wave, make_silence, write_wav

Changes

File	Action
tests/conftest.py	CREATE — fixtures, helpers, marker registration
tests/test_utils.py	CREATE — Tier 1 pure unit tests
tests/test_audio_proc.py	CREATE — Tier 2 audio processing tests
tests/test_model.py	MODIFY — moved BPM/split tests to test_utils, added architecture tests, marked slow
tests/test_predictor.py	CREATE — Tier 3+4 integration & edge cases
pyproject.toml	MODIFY — added pytest slow marker config

Verification

59 passed, 6 warnings in 3.32s

All ruff checks pass on test files.