First cut

shared/src/canonical-json.ts

@@ -0,0 +1,20 @@
+/**
+ * Canonical JSON serialization: sorted keys at every level, 2-space indent, trailing newline.
+ */
+
+export type JsonPrimitive = string | number | boolean | null;
+export type JsonValue = JsonPrimitive | JsonValue[] | { [key: string]: JsonValue };
+
+function sortKeys(value: unknown): unknown {
+  if (value === null || typeof value !== 'object') return value;
+  if (Array.isArray(value)) return value.map(sortKeys);
+  const sorted: Record<string, unknown> = {};
+  for (const key of Object.keys(value as Record<string, unknown>).sort()) {
+    sorted[key] = sortKeys((value as Record<string, unknown>)[key]);
+  }
+  return sorted;
+}
+
+export function stringifyCanonical(value: unknown): string {
+  return JSON.stringify(sortKeys(value), null, 2) + '\n';
+}