Refactor to request matching/replacement substrings from LLMs, rather than requesting a complete replacement for the full original text

tests/test_prompts.py

@@ -34,6 +34,9 @@ def test_prompt_requires_acoustically_plausible_transcription_errors():
     assert '"gestures" to "Jesters"' in prompt_text
     assert '"Lyra" to "Jesters"' in prompt_text
     assert "should be omitted" in prompt_text
+    assert "exact text span that needs replacement" in prompt_text
+    assert "replacement text for that span" in prompt_text
+    assert "Do not return corrections where original_text and corrected_text are identical" in prompt_text
 
 
 def test_prompt_uses_simplified_segment_payload():