refactor: extract voice utils to domain/voice_utils.py

- Extract infer_provider_from_spec, supertonic_voice_from_spec, split_speaker_reference, formula_from_kokoro_entry, coerce_truthy to domain/voice_utils.py
- Add tests/test_voice_utils.py with 24 tests
- All tests match old behavior

tests/test_voice_utils.py

@@ -0,0 +1,118 @@
+"""Tests for domain/voice_utils.py."""
+import sys
+import os
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from abogen.domain.voice_utils import (
+    infer_provider_from_spec,
+    supertonic_voice_from_spec,
+    split_speaker_reference,
+    formula_from_kokoro_entry,
+    coerce_truthy,
+)
+
+
+class TestInferProviderFromSpec:
+    def test_empty_returns_fallback(self):
+        assert infer_provider_from_spec("", "kokoro") == "kokoro"
+
+    def test_supertonic_uppercase(self):
+        assert infer_provider_from_spec("M1", "kokoro") == "supertonic"
+
+    def test_kokoro_voice(self):
+        assert infer_provider_from_spec("af_bella", "kokoro") == "kokoro"
+
+    def test_custom_mix(self):
+        assert infer_provider_from_spec("__custom_mix", "kokoro") == "kokoro"
+
+    def test_formula(self):
+        assert infer_provider_from_spec("af_bella*0.5+am_adam*0.5", "kokoro") == "kokoro"
+
+
+class TestSupertonicVoiceFromSpec:
+    def test_normal(self):
+        assert supertonic_voice_from_spec("m1", "m2") == "M1"
+
+    def test_empty_uses_fallback(self):
+        assert supertonic_voice_from_spec("", "m2") == "M2"
+
+    def test_formula_uses_fallback(self):
+        assert supertonic_voice_from_spec("m1*0.5", "m2") == "M2"
+
+    def test_both_empty_uses_default(self):
+        assert supertonic_voice_from_spec("", "") == "M1"
+
+
+class TestSplitSpeakerReference:
+    def test_speaker(self):
+        name, original = split_speaker_reference("speaker:John")
+        assert name == "John"
+        assert original == "speaker:John"
+
+    def test_profile(self):
+        name, original = split_speaker_reference("profile:Main")
+        assert name == "Main"
+        assert original == "profile:Main"
+
+    def test_invalid_prefix(self):
+        name, original = split_speaker_reference("voice:John")
+        assert name is None
+        assert original == "voice:John"
+
+    def test_no_colon(self):
+        name, original = split_speaker_reference("John")
+        assert name is None
+        assert original == "John"
+
+    def test_empty(self):
+        name, original = split_speaker_reference("")
+        assert name is None
+        assert original == ""
+
+
+class TestFormulaFromKokoroEntry:
+    def test_normal(self):
+        entry = {"voices": [["af_bella", 0.5], ["am_adam", 0.5]]}
+        result = formula_from_kokoro_entry(entry)
+        assert "af_bella" in result
+        assert "am_adam" in result
+
+    def test_empty(self):
+        assert formula_from_kokoro_entry({}) == ""
+
+    def test_invalid_items(self):
+        entry = {"voices": [["af_bella", "invalid"], ["am_adam", 0.5]]}
+        result = formula_from_kokoro_entry(entry)
+        assert "am_adam" in result
+        assert "af_bella" not in result
+
+
+class TestCoerceTruthy:
+    def test_bool_true(self):
+        assert coerce_truthy(True) is True
+
+    def test_bool_false(self):
+        assert coerce_truthy(False) is False
+
+    def test_string_true(self):
+        assert coerce_truthy("true") is True
+        assert coerce_truthy("yes") is True
+        assert coerce_truthy("1") is True
+        assert coerce_truthy("on") is True
+
+    def test_string_false(self):
+        assert coerce_truthy("false") is False
+        assert coerce_truthy("no") is False
+        assert coerce_truthy("0") is False
+        assert coerce_truthy("off") is False
+        assert coerce_truthy("") is False
+
+    def test_none_default_true(self):
+        assert coerce_truthy(None, True) is True
+
+    def test_none_default_false(self):
+        assert coerce_truthy(None, False) is False
+
+    def test_int(self):
+        assert coerce_truthy(1) is True
+        assert coerce_truthy(0) is False