Add H3 beat prompt builder node

tests/test_dumas_h3_beat_prompt.py

@@ -0,0 +1,46 @@
+import importlib
+import unittest
+
+
+class DumasH3BeatPromptTests(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.module = importlib.import_module("dumas_h3_beat_prompt")
+
+    def test_parse_state_falls_back_to_default(self):
+        state = self.module._parse_beat_prompt_state("not json")
+        self.assertEqual(
+            state,
+            {"beats": [{"text": "Describe this beat."}]},
+        )
+
+    def test_assemble_prompt_joins_beats_with_blank_lines(self):
+        prompt = self.module._assemble_beat_prompt(
+            {
+                "beats": [
+                    {"text": "A woman enters the room."},
+                    {"text": "wardrobe: Maya = red jacket\nShe sits at the table."},
+                    {"text": "  "},
+                    {"text": "music: low synth pulse"},
+                ]
+            }
+        )
+        self.assertEqual(
+            prompt,
+            (
+                "A woman enters the room.\n\n"
+                "wardrobe: Maya = red jacket\nShe sits at the table.\n\n"
+                "music: low synth pulse"
+            ),
+        )
+
+    def test_node_build_prompt_uses_hidden_state(self):
+        node = self.module.DumasH3BeatPromptNode()
+        result = node.build_prompt(
+            '{"beats":[{"text":"Beat one"},{"text":"Beat two"}]}'
+        )
+        self.assertEqual(result, ("Beat one\n\nBeat two",))
+
+
+if __name__ == "__main__":
+    unittest.main()