feat(topology): M2 - API-driven demo page with layout tables and dynamic rendering

- Add diagram_layouts and diagram_nodes tables
- Seed layout coordinates extracted from SVG
- Extend snapshot API to return layout data
- Rewrite demo page to fetch from API dynamically
- Support ?mission= and ?at= URL parameters
- Modal details populated from database component data
- Show/hide nodes based on component existence at snapshot time

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

topology_api/queries/snapshot_queries.py

@@ -201,3 +201,48 @@ def fetch_snapshot(snapshot_at):
                 })
 
     return result
+
+
+def fetch_layout():
+    """Fetch the default diagram layout with node positions."""
+    with read_cursor() as cur:
+        cur.execute("""
+            SELECT
+                dl.layout_key,
+                dl.name AS layout_name,
+                dl.canvas_width,
+                dl.canvas_height,
+                dn.node_key,
+                dn.component_id,
+                c.component_key,
+                dn.x, dn.y, dn.width, dn.height,
+                dn.style_key, dn.label_override, dn.z_index
+            FROM diagram_layouts dl
+            JOIN diagram_nodes dn ON dn.layout_id = dl.id
+            LEFT JOIN components c ON c.id = dn.component_id
+            WHERE dl.base_id = (SELECT id FROM bases WHERE code = 'guanghan')
+              AND dl.is_default = true
+            ORDER BY dn.z_index, dn.node_key
+        """)
+        nodes = []
+        layout_info = None
+        for row in cur.fetchall():
+            if layout_info is None:
+                layout_info = {
+                    "key": row["layout_key"],
+                    "name": row["layout_name"],
+                    "canvasWidth": float(row["canvas_width"]),
+                    "canvasHeight": float(row["canvas_height"]),
+                }
+            nodes.append({
+                "nodeKey": row["node_key"],
+                "componentKey": row["component_key"],
+                "x": float(row["x"]),
+                "y": float(row["y"]),
+                "width": float(row["width"]),
+                "height": float(row["height"]),
+                "styleKey": row["style_key"],
+                "labelOverride": row["label_override"],
+                "zIndex": row["z_index"],
+            })
+        return {"layout": layout_info, "nodes": nodes} if layout_info else None