feat(sample_features): librosa overfetch extractor + TDD on real samples

~35-feature vector (Peeters/CUIDADO timbre + MIR), convol-inspired L0→L1 tier tags.
Validated kick↔bass discriminators on real samples (temporal_centroid + decay_slope):
bd 0.064s/-165dB/s vs fbass 2.30s/-11dB/s. 5 invariant tests green.

armada/tide-table/tests/test_features.py

+"""TDD on REAL samples — the feature extractor must encode physical/domain truth.
+
+These assert RELATIVE invariants that are guaranteed by the physics of the sounds
+(a kick is a low transient; a hat is bright & noisy; a held bass sustains), so they
+stay robust as the extractor evolves. They are the ground-truth contract for the
+kick↔bass discriminators that close the #69 residual. Uses the local Dirt-Samples
+(skipped cleanly if a folder isn't present, so CI without samples still passes)."""
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+import sample_features as SF  # noqa: E402
+
+SF.MAX_FILES = 4              # 4 files/folder is enough for a stable mean, keeps tests fast
+_CACHE = {}
+
+
+def feat(folder):
+    """Folder-mean feature dict, cached. Skips the test if the folder is absent."""
+    if folder not in _CACHE:
+        if SF.folder_files(folder) is None:
+            pytest.skip(f"Dirt-Samples/{folder} not present")
+        _CACHE[folder] = SF.folder_features(folder)["mean"]
+    return _CACHE[folder]
+
+
+def test_kick_is_transient_bass_is_sustained():
+    """The #69 residual: a kick's energy is EARLY and decays FAST; a held bass's
+    energy arrives late and decays slowly. temporal_centroid + decay_slope split them."""
+    bd, fbass = feat("bd"), feat("fbass")
+    assert bd["temporal_centroid"] < 0.5 < fbass["temporal_centroid"]
+    assert bd["decay_slope_db_s"] < -50 < fbass["decay_slope_db_s"]
+
+
+def test_kick_is_lowest_register():
+    """A kick sits below melodic/pad material in both spectral centroid and f0."""
+    bd, pad = feat("bd"), feat("pad")
+    assert bd["spectral_centroid"] < pad["spectral_centroid"]
+    assert bd["f0_median"] < pad["f0_median"]
+
+
+def test_kick_more_percussive_than_pad():
+    """HPSS percussive fraction is high for a drum, low for a sustained pad."""
+    assert feat("bd")["pct_percussive"] > feat("pad")["pct_percussive"]
+
+
+def test_hat_brighter_and_noisier_than_kick():
+    """A hi-hat is high-centroid and spectrally flatter (noisier) than a sub kick."""
+    bd, hh = feat("bd"), feat("hh")
+    assert hh["spectral_centroid"] > bd["spectral_centroid"]
+    assert hh["spectral_flatness"] >= bd["spectral_flatness"]
+
+
+def test_break_loop_fires_multiple_onsets():
+    """rhythm scope: a breakbeat loop has many onsets/sec; a single kick has ~one."""
+    for brk in ("jungle_breaks", "fbreak120", "breaks165", "amen"):
+        if SF.folder_files(brk):
+            y = SF._load(SF.folder_files(brk)[0])
+            assert SF.features(y, rhythm=True).get("onset_rate", 0) > 1.0
+            return
+    pytest.skip("no break folder present")