feat(protocol): negotiate Trace v3 with v2 compatibility

Define Trace v3 wire types and generated schemas while keeping protocol
1.1 peers compatible with 1.0. Default record start still produces Trace
v2, and only an explicit trace_version=3 request enables the new shape.
Refs #90

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

crates/bsk-protocol/src/bin/dump-schema.rs

@@ -113,6 +113,7 @@ fn main() {
     dump!(RequestHelpParams, "tool_request_help_params");
     dump!(RequestHelpResult, "tool_request_help_result");
 
+    dump!(TraceV2, "trace_v2");
     dump!(Trace, "trace");
     dump!(Step, "trace_step");
     dump!(RecordStartParams, "tool_record_start_params");