refactor: consolidate voice formula building into voice_formulas.py

- voice_formulas.py: add pairs_to_formula() as canonical implementation
- webui/routes/utils/voice.py: formula_from_profile() and pairs_to_formula()
  now delegate to voice_formulas.pairs_to_formula()
- pyqt/gui.py: get_voice_formula() now uses voice_formulas.pairs_to_formula()
  instead of inline string formatting
- Eliminates 3 duplicate implementations of voice*weight formula building
- +9 tests
- 1178 tests pass

tests/test_voice_formulas_pairs.py

@@ -0,0 +1,44 @@
+"""Tests for voice_formulas.py — pairs_to_formula."""
+
+from abogen.voice_formulas import pairs_to_formula
+
+
+class TestPairsToFormula:
+    def test_basic_pair(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 1.0)])
+        assert result == "A*0.5+B*0.5"
+
+    def test_unequal_weights(self):
+        result = pairs_to_formula([("A", 3.0), ("B", 1.0)])
+        assert result == "A*0.75+B*0.25"
+
+    def test_single_voice(self):
+        result = pairs_to_formula([("A", 1.0)])
+        assert result == "A*1"
+
+    def test_filters_zero_weight(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 0.0)])
+        assert result == "A*1"
+
+    def test_all_zero_returns_none(self):
+        result = pairs_to_formula([("A", 0.0), ("B", 0.0)])
+        assert result is None
+
+    def test_empty_returns_none(self):
+        result = pairs_to_formula([])
+        assert result is None
+
+    def test_none_values_filtered(self):
+        result = pairs_to_formula([("A", 1.0), ("B", None)])
+        assert result is not None
+
+    def test_weight_normalization(self):
+        result = pairs_to_formula([("A", 2.0), ("B", 2.0)])
+        assert result == "A*0.5+B*0.5"
+
+    def test_three_voices(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 1.0), ("C", 1.0)])
+        assert "A*" in result
+        assert "B*" in result
+        assert "C*" in result
+        assert "+" in result