feat(trace): witnesses map and skipped-verification marker on Step

internal/trace/writer.go

@@ -30,6 +30,21 @@ type Step struct {
 	// retry budget. The verifier is skipped for these steps so transient
 	// state does not poison the previous/current extractor advance.
 	Transitional bool `json:"transitional,omitempty"`
+	// SkippedVerification is set true exactly when the verifier was skipped
+	// for this step, so downstream tooling can tell a deliberately-skipped
+	// step from one that was verified and came back clean.
+	SkippedVerification bool `json:"skipped_verification,omitempty"`
+	// Witnesses records the violation witness for each property that newly
+	// violated at this step: the cause and the extractor values at onset.
+	Witnesses map[string]Witness `json:"witnesses,omitempty"`
+}
+
+// Witness is the trace-side record of a property violation: why it fired and a
+// snapshot of every extractor's value at the violating step.
+type Witness struct {
+	Reason     string                     `json:"reason,omitempty"`
+	IsError    bool                       `json:"is_error,omitempty"`
+	Extractors map[string]json.RawMessage `json:"extractors,omitempty"`
 }
 
 // ExtractorChange records the prev/curr JSON values of an extractor whose