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/src/tools/record_v2.rs

@@ -75,6 +75,11 @@ pub enum StepV2 {
         common: StepCommonV2,
         target: TargetDescriptorV2,
     },
+    Hover {
+        #[serde(flatten)]
+        common: StepCommonV2,
+        target: TargetDescriptorV2,
+    },
     Fill {
         #[serde(flatten)]
         common: StepCommonV2,
@@ -102,6 +107,7 @@ pub enum StepV2 {
 
 /// Persisted user-action trace exported by legacy `tool.record_stop` / `await`.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
+#[serde(deny_unknown_fields)]
 pub struct TraceV2 {
     /// RFC 3339 timestamp when recording stopped.
     pub recorded_at: String,
@@ -197,7 +203,7 @@ mod tests {
     }
 
     #[test]
-    fn historical_v2_rejects_hover_steps() {
+    fn v2_hover_steps_round_trip() {
         let value = json!({
             "recorded_at": "2026-07-21T08:00:00Z",
             "entry": { "start_url": "https://example.com/" },
@@ -206,10 +212,15 @@ mod tests {
                 "op": "hover",
                 "id": 1,
                 "page": "p1",
-                "target": { "tag": "button", "name": "结束" }
+                "target": { "tag": "span", "role": "button", "name": "Account" }
             }]
         });
 
-        assert!(serde_json::from_value::<TraceV2>(value).is_err());
+        let trace: TraceV2 = serde_json::from_value(value).unwrap();
+        assert!(matches!(trace.steps.as_slice(), [StepV2::Hover { .. }]));
+        assert_eq!(
+            serde_json::to_value(&trace).unwrap()["steps"][0]["op"],
+            "hover"
+        );
     }
 }