feat: Add WorldGraph for deterministic room descriptions (closes #155)

Implements Python-side room graph data structures for LLM agent environments:
- Room, Door, WorldObject dataclasses with full metadata
- WorldGraph class with spatial queries (room_at, get_exits)
- Deterministic text generation (describe_room, describe_exits)
- Available action enumeration based on room state
- Factory functions for test scenarios (two_room, button_door)

Example output:
"You are in the guard room. The air is musty. On the ground you see
a brass key. Exits: east (the armory)."

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/vllm_demo/test_world_graph.py

@@ -0,0 +1,139 @@
+"""
+Unit tests for WorldGraph
+"""
+
+from world_graph import (
+    WorldGraph, Room, Door, WorldObject, Direction,
+    AgentInfo, create_two_room_scenario, create_button_door_scenario
+)
+
+def test_room_contains():
+    """Test room boundary checking."""
+    room = Room("test", "test room", bounds=(5, 5, 10, 10))
+
+    assert room.contains(5, 5) == True   # Top-left corner
+    assert room.contains(14, 14) == True # Bottom-right (exclusive)
+    assert room.contains(15, 15) == False # Outside
+    assert room.contains(4, 5) == False  # Just outside left
+
+    print("PASS: room_contains")
+
+def test_room_at():
+    """Test spatial room lookup."""
+    world = create_two_room_scenario()
+
+    # Guard room is at (1,1) with size (8,8)
+    room = world.room_at(3, 3)
+    assert room is not None
+    assert room.name == "guard_room"
+
+    # Armory is at (11,1) with size (8,8)
+    room = world.room_at(13, 3)
+    assert room is not None
+    assert room.name == "armory"
+
+    # Between rooms (the door area) - should return None
+    room = world.room_at(9, 4)
+    assert room is None
+
+    print("PASS: room_at")
+
+def test_describe_room_basic():
+    """Test basic room description."""
+    world = create_two_room_scenario()
+
+    desc = world.describe_room("guard_room")
+
+    assert "You are in the guard room" in desc
+    assert "brass key" in desc
+    assert "Exits:" in desc
+    assert "east" in desc
+    assert "armory" in desc
+
+    print("PASS: describe_room_basic")
+    print(f"  Output: {desc}")
+
+def test_describe_room_with_agents():
+    """Test room description with visible agents."""
+    world = create_two_room_scenario()
+
+    agents = [
+        AgentInfo("Wizard", "a wizard", (3, 3)),
+        AgentInfo("Knight", "a knight", (4, 4)),
+    ]
+
+    desc = world.describe_room("guard_room", visible_agents=agents, observer_name="Wizard")
+
+    assert "knight" in desc.lower()
+    assert "wizard" not in desc.lower()  # Observer excluded
+
+    print("PASS: describe_room_with_agents")
+    print(f"  Output: {desc}")
+
+def test_describe_locked_door():
+    """Test that locked doors are described correctly."""
+    world = create_button_door_scenario()
+
+    desc = world.describe_room("button_room")
+
+    assert "locked" in desc.lower()
+
+    print("PASS: describe_locked_door")
+    print(f"  Output: {desc}")
+
+def test_available_actions():
+    """Test action enumeration."""
+    world = create_two_room_scenario()
+
+    actions = world.get_available_actions("guard_room")
+
+    assert "GO EAST" in actions
+    assert "TAKE brass_key" in actions
+    assert "LOOK" in actions
+    assert "WAIT" in actions
+
+    print("PASS: available_actions")
+    print(f"  Actions: {actions}")
+
+def test_determinism():
+    """Test that descriptions are deterministic."""
+    world = create_two_room_scenario()
+
+    desc1 = world.describe_room("guard_room")
+    desc2 = world.describe_room("guard_room")
+    desc3 = world.describe_room("guard_room")
+
+    assert desc1 == desc2 == desc3, "Descriptions must be deterministic!"
+
+    print("PASS: determinism")
+
+def test_direction_opposites():
+    """Test direction opposite calculation."""
+    assert Direction.NORTH.opposite == Direction.SOUTH
+    assert Direction.SOUTH.opposite == Direction.NORTH
+    assert Direction.EAST.opposite == Direction.WEST
+    assert Direction.WEST.opposite == Direction.EAST
+
+    print("PASS: direction_opposites")
+
+def run_all_tests():
+    """Run all WorldGraph tests."""
+    print("=" * 50)
+    print("WorldGraph Unit Tests")
+    print("=" * 50)
+
+    test_room_contains()
+    test_room_at()
+    test_describe_room_basic()
+    test_describe_room_with_agents()
+    test_describe_locked_door()
+    test_available_actions()
+    test_determinism()
+    test_direction_opposites()
+
+    print("=" * 50)
+    print("All tests passed!")
+    print("=" * 50)
+
+if __name__ == "__main__":
+    run_all_tests()