introduce tags, structs, and reticles

this was meant to be split into smaller changes, but I realised I edited my existing revision too late.

static/session.js

@@ -267,22 +267,13 @@ class Session extends EventTarget {
         });
     }
 
-    sendPlot(points) {
+    sendInteraction(interactions) {
+        console.log(interactions);
         this.#sendJson({
             request: "wall",
             wallEvent: {
-                event: "plot",
-                points,
-            },
-        });
-    }
-
-    sendSetBrush(brush) {
-        this.#sendJson({
-            request: "wall",
-            wallEvent: {
-                event: "setBrush",
-                brush,
+                event: "interact",
+                interactions,
             },
         });
     }