Harden chunk map export and retire completed plans

internal/modules/generic/output/json/encoder.go

@@ -1,9 +1,11 @@
 package json
 
 import (
+	"bytes"
 	"context"
 	stdjson "encoding/json"
 	"fmt"
+	"io"
 	"mime"
 	"regexp"
 	"sort"
@@ -247,10 +249,16 @@ func serializedOutputFile(name string, artifact contracts.SerializedArtifact) (c
 	if !isJSONMediaType(mediaType) {
 		return contracts.OutputFile{}, encoderErrorf("normalized output %q has unsupported media type %q", name, mediaType)
 	}
+	decoder := stdjson.NewDecoder(bytes.NewReader(content))
+	decoder.UseNumber()
 	var decoded any
-	if err := stdjson.Unmarshal(content, &decoded); err != nil {
+	if err := decoder.Decode(&decoded); err != nil {
 		return contracts.OutputFile{}, encoderErrorf("normalized output %q contains invalid JSON: %w", name, err)
 	}
+	var trailing any
+	if err := decoder.Decode(&trailing); err != io.EOF {
+		return contracts.OutputFile{}, encoderErrorf("normalized output %q contains multiple JSON values", name)
+	}
 	pretty, err := marshalPretty(decoded)
 	if err != nil {
 		return contracts.OutputFile{}, err