Fixed rule conflict management. Added User guide for formatting

tests/core/formatting/test_engine.py

@@ -215,6 +215,75 @@ class TestConflictResolution:
 
         assert "color: red" in css
 
+    def test_style_and_formatter_fusion(self):
+        """
+        Test that style and formatter can come from different rules.
+
+        Scenario:
+        - Rule 1: format("EUR") - unconditional formatter
+        - Rule 2: style("secondary") if value > col.X - conditional style
+
+        When condition is met:
+        - Style from Rule 2 (higher specificity for style)
+        - Formatter from Rule 1 (only rule with formatter)
+        - Both should be applied (fusion)
+        """
+        engine = FormattingEngine()
+        rules = [
+            FormatRule(formatter=NumberFormatter(precision=2, suffix=" €")),  # unconditional
+            FormatRule(
+                condition=Condition(operator=">", value={"col": "budget"}),
+                style=Style(preset="secondary")  # conditional
+            ),
+        ]
+        row_data = {"budget": 100}
+
+        # Case 1: Condition met (value > budget)
+        css, formatted = engine.apply_format(rules, cell_value=150, row_data=row_data)
+
+        assert css is not None
+        assert "var(--color-secondary)" in css  # Style from Rule 2
+        assert formatted == "150.00 €"  # Formatter from Rule 1
+
+        # Case 2: Condition not met (value <= budget)
+        css, formatted = engine.apply_format(rules, cell_value=50, row_data=row_data)
+
+        assert css is None  # No style (Rule 2 doesn't match)
+        assert formatted == "50.00 €"  # Formatter from Rule 1 still applies
+
+    def test_multiple_styles_and_formatters_highest_specificity_wins(self):
+        """
+        Test that style and formatter are resolved independently with specificity.
+
+        Rules:
+        - Rule 1: style("neutral") - unconditional
+        - Rule 2: format("EUR") - unconditional
+        - Rule 3: style("error") if value < 0 - conditional style
+        - Rule 4: format.number(precision=0) if value < 0 - conditional formatter
+
+        When value < 0:
+        - Style from Rule 3 (higher specificity)
+        - Formatter from Rule 4 (higher specificity)
+        """
+        engine = FormattingEngine()
+        rules = [
+            FormatRule(style=Style(preset="neutral")),
+            FormatRule(formatter=NumberFormatter(precision=2, suffix=" €")),
+            FormatRule(
+                condition=Condition(operator="<", value=0),
+                style=Style(preset="error")
+            ),
+            FormatRule(
+                condition=Condition(operator="<", value=0),
+                formatter=NumberFormatter(precision=0, suffix=" €")
+            ),
+        ]
+
+        css, formatted = engine.apply_format(rules, cell_value=-5.67)
+
+        assert "var(--color-error)" in css  # Rule 3 wins for style
+        assert formatted == "-6 €"  # Rule 4 wins for formatter (precision=0)
+
 
 class TestWithRowData:
     def test_condition_with_column_reference(self):