fix: cross-tool audit findings + alignment with format standardizer

Closes 12 bugs and 8 gaps surfaced by parallel audits across all core
modules, plus aligns the dedup-side normalizers with the new
format_standardize behavior where they had silently diverged.

Bugs (data integrity / correctness):
- dedup: NaN/None values matched as duplicates because str(None)='None'.
  Two rows with missing email silently merged.
- dedup: removed_df had 0 columns when nothing was removed; downstream
  code expecting matching schema broke. Now preserves column shape.
- dedup: ColumnMatchStrategy threshold accepted any value; out-of-range
  silently broke matching. Validated to [0, 100] in __post_init__.
- dedup: strategy referencing a missing column was silently skipped.
  Now raises ValueError listing available columns.
- fixes: replace_null_sentinels crashed on non-string sentinels (int/None
  from JSON payload). Coerced to str.
- fixes: _vectorized_regex_sub raised raw re.error on bad patterns. Now
  wraps as ValueError with clear message.
- io: detect_header_row mis-identified all-empty and metadata-only rows
  as headers (all([]) is True). Now requires ≥2 non-empty cells.
- config: from_dict crashed when JSON had unknown fields, breaking
  forward compat. Now filters to known fields.
- analyze: mixed-case email detector flagged all-None columns because
  str(None)='None' contains both N and one. Now drops NaN before stringify.

New features and gap closures:
- io: _detect_excel_header_row mirrors detect_header_row for Excel via
  openpyxl read-only; _read_excel uses it when header_row=None.
- io: write_file gains delimiter + encoding params; .tsv extension
  defaults to tab.
- normalizers: normalize_phone preserves extensions as ;ext=N suffix.
- normalizers: normalize_address folds spelled-out US state names to
  2-letter codes (California ≡ CA).
- normalizers: normalize_name drops surname particles (van, de, von)
  so "Charles de Gaulle" ≡ "Charles Gaulle" for matching.
- analyze: new _detect_inconsistent_date_format detector flags columns
  with mixed ISO/US/EU date shapes; routes to format standardizer.
- analyze: _NULL_LIKE recognizes "<na>" (pd.NA repr).
- analyze: duplicate-row finding renamed count → n_extra (rows that
  would actually be removed) with clarified description.
- dedup: group_confidence no longer falsely 100.0 when transitive group
  members lack a recorded direct pair; falls back to 100.0 only when
  truly no pairs were observed.
- dedup: MatchResult / DeduplicationResult docstrings clarify that
  row_indices refer to the input frame's positional index (output index
  is reset).
- text_clean: visualize_hidden_html(None) now returns None (matches
  visualize_hidden_text); strip_bom strips at most one BOM per call;
  sentence_case dead elif branch removed.

Tests:
- tests/test_audit_fixes.py — 28 regression tests, one or more per
  numbered finding, named after BUG/GAP/NIT tags so future readers
  can trace each test back to its audit.
- tests/test_fixes_unit.py — 26 isolated unit tests for previously
  integration-only fix functions (trim_whitespace, strip_nbsp,
  strip_zero_width, normalize_line_endings, clean_headers,
  repair_mojibake — last skipped if ftfy unavailable).
- tests/test_io.py — adds CSV / TSV / semicolon / UTF-8-BOM round-trip
  tests + Excel auto-header-detection tests.
- tests/test_normalizers.py — adds 8 tests for the alignment work
  above (phone extension, state names, particles).

Adds .claude/ to .gitignore (agent worktrees + local settings).

Full project suite: 1197 passed, 4 skipped, 17 xfailed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_io.py

@@ -261,3 +261,78 @@ class TestReadCsvRepaired:
         df, repair = read_csv_repaired(f)
         assert len(df) == 2
         assert repair.changed is False
+
+
+# ---------------------------------------------------------------------------
+# Round-trip integrity (audit GAP-19, GAP-21)
+# ---------------------------------------------------------------------------
+
+class TestRoundTrip:
+    def test_csv_roundtrip_preserves_values(self, tmp_path):
+        df = pd.DataFrame({
+            "id": ["1", "2", "3"],
+            "name": ["Alice", "Bob", "Carol"],
+            "amount": ["10.50", "20.25", "30.00"],
+        })
+        path = tmp_path / "rt.csv"
+        write_file(df, path)
+        loaded = read_file(path)
+        assert list(loaded.columns) == list(df.columns)
+        assert len(loaded) == len(df)
+        for col in df.columns:
+            assert list(loaded[col]) == list(df[col])
+
+    def test_tsv_roundtrip_via_extension(self, tmp_path):
+        df = pd.DataFrame({"a": ["1", "2"], "b": ["x", "y, z"]})
+        path = tmp_path / "rt.tsv"
+        write_file(df, path)
+        # Confirm tab is used and embedded comma in 'b' survives.
+        loaded = read_file(path)
+        assert list(loaded.columns) == ["a", "b"]
+        assert loaded.iloc[1]["b"] == "y, z"
+
+    def test_semicolon_roundtrip_via_explicit_delimiter(self, tmp_path):
+        df = pd.DataFrame({"a": ["1", "2"], "b": ["x", "y"]})
+        path = tmp_path / "rt.csv"
+        write_file(df, path, delimiter=";")
+        loaded = read_file(path)
+        assert list(loaded.columns) == ["a", "b"]
+        assert loaded.iloc[0]["a"] == "1"
+
+    def test_utf8_bom_non_ascii_roundtrip(self, tmp_path):
+        df = pd.DataFrame({"name": ["café", "naïve", "résumé"]})
+        path = tmp_path / "utf8.csv"
+        write_file(df, path)
+        loaded = read_file(path)
+        assert list(loaded["name"]) == ["café", "naïve", "résumé"]
+
+
+class TestExcelHeaderDetection:
+    def test_excel_with_metadata_rows(self, tmp_path):
+        from openpyxl import Workbook
+        wb = Workbook()
+        ws = wb.active
+        # Two leading blank rows + header + data.
+        ws.append(["Report generated 2024-01-15", None, None])
+        ws.append([None, None, None])
+        ws.append(["name", "email", "phone"])
+        ws.append(["alice", "a@x.com", "555-1234"])
+        ws.append(["bob", "b@x.com", "555-5678"])
+        path = tmp_path / "report.xlsx"
+        wb.save(path)
+        df = read_file(path)
+        # Auto-detected header row 2 → columns are name/email/phone
+        assert list(df.columns) == ["name", "email", "phone"]
+        assert len(df) == 2
+
+    def test_excel_normal_header_row_zero(self, tmp_path):
+        from openpyxl import Workbook
+        wb = Workbook()
+        ws = wb.active
+        ws.append(["name", "email"])
+        ws.append(["alice", "a@x.com"])
+        path = tmp_path / "normal.xlsx"
+        wb.save(path)
+        df = read_file(path)
+        assert list(df.columns) == ["name", "email"]
+        assert len(df) == 1