feat(text_clean): preserve internal whitespace in numeric/date/phone cells

Closes the §4.17 spec gap that test_gap_coverage.py was tracking via xfail:
collapse_whitespace must NOT touch cells whose shape carries meaningful
internal whitespace.

Adds _looks_structured(s) — returns True when s matches:
  - numeric (currency optional, thousand-grouping by , . or single space)
  - date (ISO/slash/dot separator, or 'Mon DD YYYY' / 'DD Mon YYYY')
  - phone (digits + parens/dots/dashes/+/spaces, >= 7 digits, no letters)

The pipeline uses a new _smart_collapse_whitespace wrapper that defers to
collapse_whitespace only when _looks_structured returns False. The raw
collapse_whitespace function is unchanged so direct callers and existing
unit tests remain valid.

Five new positive tests replace the xfail:
  - "(555)  123-4567" preserved (phone, double space inside)
  - "1 234" preserved (European thousands)
  - "2024-01-15" preserved (ISO date)
  - "Jan 15 2024" preserved (textual date)
  - "hello  world" still collapsed to "hello world" (free-text negative case)

Conservative on purpose: a false negative just collapses (existing
behavior); a false positive leaves intentional double spaces in prose.

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

tests/test_gap_coverage.py

@@ -137,25 +137,31 @@ class TestFindingsByToolEdges:
 # Known gap: collapse_whitespace on numeric/date/phone-shaped cells
 # ---------------------------------------------------------------------------
 
-class TestNumericPhoneWhitespaceGap:
-    """Spec §4.17: ``collapse_whitespace`` should NOT collapse internal
-    whitespace in cells that look numeric, dated, or phone-shaped.
+class TestStructuredCellWhitespacePreservation:
+    """Spec §4.17: ``collapse_whitespace`` skips numeric/date/phone-shaped cells."""
 
-    Currently unconditional. Marked xfail so the suite tracks the gap
-    without silently allowing regressions on the cells that *do* get
-    correctly collapsed.
-    """
-
-    @pytest.mark.xfail(
-        reason=(
-            "Heuristic not yet implemented — collapse_whitespace runs on every "
-            "string cell, including phone-shaped ones. See TEST-CASES.md §4.17."
-        ),
-        strict=True,
-    )
     def test_phone_internal_double_space_preserved(self):
-        df = pd.DataFrame({"phone": ["(555)  123-4567"]})  # double space inside
+        df = pd.DataFrame({"phone": ["(555)  123-4567"]})
         result = clean_dataframe(df)
-        # Spec requires the double space to survive because the cell looks
-        # phone-shaped. Today the cleaner collapses it.
         assert result.cleaned_df.iloc[0]["phone"] == "(555)  123-4567"
+
+    def test_european_thousands_sep_preserved(self):
+        df = pd.DataFrame({"price": ["1 234"]})
+        result = clean_dataframe(df)
+        assert result.cleaned_df.iloc[0]["price"] == "1 234"
+
+    def test_iso_date_passes_through(self):
+        df = pd.DataFrame({"date": ["2024-01-15"]})
+        result = clean_dataframe(df)
+        assert result.cleaned_df.iloc[0]["date"] == "2024-01-15"
+
+    def test_textual_date_preserves_spaces(self):
+        df = pd.DataFrame({"date": ["Jan 15 2024"]})
+        result = clean_dataframe(df)
+        assert result.cleaned_df.iloc[0]["date"] == "Jan 15 2024"
+
+    def test_free_text_double_space_still_collapsed(self):
+        # Crucially, the heuristic must NOT trigger on prose with letters.
+        df = pd.DataFrame({"note": ["hello  world"]})
+        result = clean_dataframe(df)
+        assert result.cleaned_df.iloc[0]["note"] == "hello world"