refactor: remove exp_data experimental data package

Detector data handling, energy calibration, peak fitting and
instrumental corrections leave the project scope (theory-only
spectrum shape calculation). Removes the package, its tests, the
exp pipeline integration test, the calibration notebook, and the
ExpSpectrum fixtures from the shared conftest.

tests/conftest.py

@@ -3,9 +3,6 @@
 import numpy as np
 import pytest
 
-from exp_data.spectrum import ExpSpectrum
-from exp_data.calibration import CalibrationResult
-
 
 @pytest.fixture()
 def W_low():
@@ -38,47 +35,6 @@ def large_W():
     return np.linspace(1.002, 4.9, 1000)
 
 
-@pytest.fixture()
-def synthetic_spectrum():
-    """Create a synthetic ExpSpectrum (4096 channels)."""
-    n_channels = 4096
-    energies = np.arange(n_channels, dtype=np.float64)
-    counts = np.exp(-energies / 500) * 1000 + np.random.poisson(5, n_channels)
-    return ExpSpectrum(
-        energies=energies,
-        counts=counts,
-        source="Tc99",
-        run_id="synthetic",
-    )
 
 
-@pytest.fixture()
-def calibration_result():
-    """Create a synthetic CalibrationResult (linear, E = 0.4 + 0.1*ch)."""
-    coeffs = np.array([0.1, 0.4])  # polyfit order: [slope, intercept]
-    cov = np.eye(2) * 1e-6
-    return CalibrationResult(
-        coefficients=coeffs,
-        order=1,
-        chi2_per_dof=0.5,
-        n_points=2,
-        covariance=cov,
-    )
 
-
-@pytest.fixture()
-def full_spectrum():
-    """Create a full ExpSpectrum with all fields populated."""
-    return ExpSpectrum(
-        energies=np.arange(4096, dtype=np.float64),
-        counts=np.exp(-np.arange(4096) / 500) * 1000 + np.random.poisson(5, 4096),
-        source="Tc99",
-        run_id="run_042",
-        date="2025-01-15",
-        dead_time=0.05,
-        live_time=3420.0,
-        metadata={
-            "calibration": {"type": "linear", "coefficients": [0.1, 0.4]},
-            "livetime": {"fraction": 0.95, "method": "pulser"},
-        },
-    )