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

abogen/voice_formulas.py

@@ -1,5 +1,5 @@
 import re
-from typing import List, Tuple
+from typing import Iterable, List, Optional, Tuple
 
 from abogen.tts_plugin.utils import get_voices
 
@@ -72,6 +72,33 @@ def parse_voice_formula(pipeline, formula):
     return weighted_sum
 
 
+def pairs_to_formula(pairs: Iterable[Tuple[str, float]]) -> Optional[str]:
+    """Build a voice formula string from (voice_name, weight) pairs.
+
+    Normalizes weights to sum to 1.0 and formats as "voice1*0.5+voice2*0.5".
+
+    Args:
+        pairs: Iterable of (voice_name, weight) tuples. Zero-weight entries
+            are filtered out.
+
+    Returns:
+        Formula string, or None if no valid entries.
+    """
+    voices = [(voice, float(weight)) for voice, weight in pairs if weight is not None and float(weight) > 0]
+    if not voices:
+        return None
+    total = sum(weight for _, weight in voices)
+    if total <= 0:
+        return None
+
+    def _format_value(value: float) -> str:
+        normalized = value / total if total else 0.0
+        return (f"{normalized:.4f}").rstrip("0").rstrip(".") or "0"
+
+    parts = [f"{voice}*{_format_value(weight)}" for voice, weight in voices]
+    return "+".join(parts)
+
+
 def calculate_sum_from_formula(formula):
     weights = re.findall(r"\* *([\d.]+)", formula)
     total_sum = sum(float(weight) for weight in weights)