fix(protocol): keep generic trace schema compatible with v2 and v3

Split standalone v2/v3 schemas and drop TraceV2 deny_unknown_fields so legacy traces with extra keys still parse; mix and version checks stay in RecordedTrace.

crates/bsk-protocol/schema/trace_v2.json

@@ -1,7 +1,7 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "title": "TraceV2",
-  "description": "Persisted user-action trace exported by legacy `tool.record_stop` / `await`.",
+  "description": "Persisted user-action trace exported by legacy `tool.record_stop` / `await`.\n\nUnknown extension fields are ignored so older traces remain readable. Mixed v2/v3 envelopes are rejected by `RecordedTrace` classification.",
   "type": "object",
   "required": [
     "entry",
@@ -42,7 +42,6 @@
       }
     }
   },
-  "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`).",