fix(protocol): align Trace v2/v3 classification and hover steps

Require Trace v3 version const 3, reject mixed pages/states payloads, and
align TypeScript selection optionality with the generated schema. Trace v2
on main already records hover steps, so the frozen v2 schema must accept
them instead of dropping them during reduction.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/bsk-protocol/schema/trace_v2.json

@@ -42,6 +42,7 @@
       }
     }
   },
+  "additionalProperties": false,
   "definitions": {
     "KeyModifier": {
       "description": "Keyboard modifier flags. Multiple flags may be combined; the extension folds them into CDP's bitfield (`alt=1, ctrl=2, meta=4, shift=8`).",
@@ -189,6 +190,46 @@
             }
           }
         },
+        {
+          "description": "Fields shared by every v2 step variant (flattened in JSON).",
+          "type": "object",
+          "required": [
+            "id",
+            "op",
+            "page",
+            "target"
+          ],
+          "properties": {
+            "effect": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/StepEffectV2"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "id": {
+              "type": "integer",
+              "format": "uint32",
+              "minimum": 0.0
+            },
+            "op": {
+              "type": "string",
+              "enum": [
+                "hover"
+              ]
+            },
+            "page": {
+              "description": "Reference into `pages[]`.",
+              "type": "string"
+            },
+            "target": {
+              "$ref": "#/definitions/TargetDescriptorV2"
+            }
+          }
+        },
         {
           "description": "Fields shared by every v2 step variant (flattened in JSON).",
           "type": "object",