Fix Cassandra schema and graph filter semantics (#680)

Schema fix (dtype/lang clustering key):
- Add dtype and lang to PRIMARY KEY in quads_by_entity table
- Add otype, dtype, lang to PRIMARY KEY in quads_by_collection table
- Fixes deduplication bug where literals with same value but different
  datatype or language tag were collapsed (e.g., "thing" vs "thing"@en)
- Update delete_collection to pass new clustering columns
- Update tech spec to reflect new schema

Graph filter semantics (simplified, no wildcard constant):
- g=None means all graphs (no filter)
- g="" means default graph only
- g="uri" means specific named graph
- Remove GRAPH_WILDCARD usage from EntityCentricKnowledgeGraph
- Fix service.py streaming and non-streaming paths
- Fix CLI to preserve empty string for -g '' argument

tests/unit/test_direct/test_entity_centric_kg.py

@@ -305,9 +305,8 @@ class TestEntityCentricKnowledgeGraph:
 
         mock_session.execute.assert_called()
 
-    def test_graph_wildcard_returns_all_graphs(self, entity_kg):
-        """Test that g='*' returns quads from all graphs"""
-        from trustgraph.direct.cassandra_kg import GRAPH_WILDCARD
+    def test_graph_none_returns_all_graphs(self, entity_kg):
+        """Test that g=None returns quads from all graphs"""
         kg, mock_session = entity_kg
 
         mock_result = [
@@ -320,7 +319,7 @@ class TestEntityCentricKnowledgeGraph:
         ]
         mock_session.execute.return_value = mock_result
 
-        results = kg.get_s('test_collection', 'http://example.org/Alice', g=GRAPH_WILDCARD)
+        results = kg.get_s('test_collection', 'http://example.org/Alice', g=None)
 
         # Should return quads from both graphs
         assert len(results) == 2