Add lucidia_core/model.py with model skeleton

lucidia_llm/lucidia_core/model.py

@@ -0,0 +1,9 @@
+"""Lucidia model definition: simple decoder-only transformer skeleton."""
+
+class TransformerModel:
+    def __init__(self, config):
+        self.config = config
+
+    def forward(self, x):
+        """Forward pass placeholder."""
+        raise NotImplementedError("Model forward pass not implemented")