feat: add documentation, Streamlit GUI, and full source tree

- Rewrite README.md with project overview, quick-start, and CLI summary
- Add docs/CLI-REFERENCE.md with full flag reference and 8 recipe sections
- Add docs/DEVELOPER.md with architecture, data flow, and extension guides
- Rewrite src/core/__init__.py with public API exports and module docstring
- Add Streamlit GUI (src/gui/) with file upload, advanced options, interactive
  match group review with side-by-side diff, and download buttons
- Add .gitignore, requirements.txt, all source code, tests, and sample data
- Add streamlit to requirements.txt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,47 @@
+"""Shared test fixtures."""
+
+import pandas as pd
+import pytest
+from pathlib import Path
+
+SAMPLES_DIR = Path(__file__).parent.parent / "samples"
+
+
+@pytest.fixture
+def sample_csv_path():
+    return SAMPLES_DIR / "messy_sales.csv"
+
+
+@pytest.fixture
+def sample_df(sample_csv_path):
+    return pd.read_csv(sample_csv_path, dtype=str, keep_default_na=False)
+
+
+@pytest.fixture
+def simple_df():
+    """Small DataFrame with obvious duplicates for unit testing."""
+    return pd.DataFrame({
+        "name": ["Alice", "alice", "Bob", "Charlie", "ALICE"],
+        "email": ["alice@test.com", "alice@test.com", "bob@test.com",
+                  "charlie@test.com", "alice@test.com"],
+        "phone": ["555-1234", "555-1234", "555-5678", "555-9012", "555-1234"],
+    })
+
+
+@pytest.fixture
+def merge_df():
+    """DataFrame with partial records that benefit from merge."""
+    return pd.DataFrame({
+        "name": ["John Doe", "John Doe", "Jane Smith"],
+        "email": ["john@test.com", "john@test.com", "jane@test.com"],
+        "phone": ["555-1111", "", "555-3333"],
+        "address": ["", "123 Main St", "456 Oak Ave"],
+    })
+
+
+@pytest.fixture
+def tmp_csv(tmp_path, simple_df):
+    """Write simple_df to a temp CSV and return the path."""
+    path = tmp_path / "test_input.csv"
+    simple_df.to_csv(path, index=False)
+    return path