Separate explanation replay snapshots and rendering

snapshot.go

@@ -0,0 +1,63 @@
+package configkit
+
+import (
+	"bytes"
+	"slices"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+// Snapshot projection is the shared redaction boundary for effective YAML
+// and explanation values. It borrows frozen trees and returns a detached tree.
+func valueSnapshot(root *document.Node, path Path, secrets map[string]bool) ValueSnapshot {
+	node, present := documentAtPath(root, path)
+	if !present {
+		return ValueSnapshot{}
+	}
+	safe, redacted := safeSnapshotDocument(node, secrets, path)
+	var output bytes.Buffer
+	if err := yamladapter.Write(&output, safe); err != nil {
+		return ValueSnapshot{Present: true, Redacted: true, YAML: "'[REDACTED]'\n"}
+	}
+	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}
+}
+
+func safeSnapshotDocument(node *document.Node, secrets map[string]bool, path Path) (*document.Node, bool) {
+	if node == nil {
+		return nil, false
+	}
+	if pathIsSecret(secrets, path) {
+		return &document.Node{Kind: document.String, Text: "[REDACTED]"}, true
+	}
+	result := *node
+	redacted := false
+	switch node.Kind {
+	case document.Mapping:
+		result.Mapping = make([]document.Entry, len(node.Mapping))
+		copy(result.Mapping, node.Mapping)
+		sortDocumentMapping(result.Mapping)
+		for index := range result.Mapping {
+			entry := &result.Mapping[index]
+			entry.Value, redacted = safeSnapshotChild(entry.Value, secrets, path.Key(entry.Key), redacted)
+		}
+	case document.Sequence:
+		result.Sequence = make([]*document.Node, len(node.Sequence))
+		for index, child := range node.Sequence {
+			result.Sequence[index], redacted = safeSnapshotChild(child, secrets, path.Index(index), redacted)
+		}
+	}
+	return &result, redacted
+}
+
+func safeSnapshotChild(node *document.Node, secrets map[string]bool, path Path, prior bool) (*document.Node, bool) {
+	safe, redacted := safeSnapshotDocument(node, secrets, path)
+	return safe, prior || redacted
+}
+
+func sortDocumentMapping(entries []document.Entry) {
+	slices.SortFunc(entries, func(left, right document.Entry) int {
+		return strings.Compare(left.Key, right.Key)
+	})
+}