Constant SCHEMA_STRING 

pub const SCHEMA_STRING: &str = "\
katra-trace-v1\n\
header: magic[8] u32 version u32 header_len bincode(TraceHeader)\n\
records: repeated u32 len + bincode(TraceRecord)\n\
TraceHeader: format_version u32, schema_hash u64, tool_version string,\n\
  capture_options CaptureOptions, start_wall_ns u64, start_mono_ns u64,\n\
  process_id u64, workload string, host HostInfo, notes Vec<string>\n\
TraceRecord: Event(TraceEvent) | EpochMarker{label, ts_mono_ns, ts_wall_ns}\n\
  | Counter{name, value, ts_mono_ns} | SessionSummary(TraceSummary)\n\
TraceEvent: seq, ts_mono_ns, ts_wall_ns, thread_id, process_id, scope, kind,\n\
  phase, span_id, request_id, causes Vec<u64>, resource, payload,\n\
  cost_estimate_ns, confidence\n\
Enums serialized by variant index; append-only evolution.\n";

A canonical description of the v1 schema. Changing this string must change SCHEMA_HASH, which forces old readers to notice.