refactor: rename visualize module to output module and separate concerns into core, csv_export, and visualization modules

tests/physics/test_multi_branch.py

@@ -33,7 +33,7 @@ from beta_spectrum import (
     BranchConfig,
     SpectrumConfig,
 )
-from beta_spectrum.visualize import BetaSpectrumAnalyzer
+from beta_spectrum.output import BetaSpectrumAnalyzer
 from beta_spectrum.utils import T_to_W
 
 # ---------------------------------------------------------------------------
@@ -461,7 +461,7 @@ class TestMultiBranchCSVExport:
         import pandas as pd
 
         # Read CSV (skip comment headers, use comma delimiter by default)
-        df = pd.read_csv(csv_path, comment="#", sep=',')
+        df = pd.read_csv(csv_path, comment="#", sep=",")
 
         # Should have total spectrum column
         assert "total_spectrum" in df.columns
@@ -500,7 +500,7 @@ class TestMultiBranchCSVExport:
 
         import pandas as pd
 
-        df = pd.read_csv(csv_path, comment="#", sep=',')
+        df = pd.read_csv(csv_path, comment="#", sep=",")
 
         assert "total_spectrum" in df.columns
         assert "branch_1_spectrum" in df.columns