Add compact JSON formatter with format-on-paste (§03A)

src/core/json-format.ts

@@ -0,0 +1,41 @@
+/**
+ * JSON pretty-printer for Vega-Lite specs (spec §03A; docs/architecture/08 →
+ * "compact formatter").
+ *
+ * Uses `json-stringify-pretty-compact` for Vega's house style — arrays/objects
+ * stay on one line until they exceed a width budget, then wrap — which reads far
+ * better for VL specs than `JSON.stringify(…, null, 2)`. Pure and portable: no
+ * Monaco, no browser. The Monaco formatting adapter
+ * (`infrastructure/monaco-format`) is a thin wrapper over this.
+ */
+
+import stringify from 'json-stringify-pretty-compact';
+
+/** Width budget before an array/object wraps onto multiple lines. */
+export const DEFAULT_MAX_LINE = 80;
+
+/** Default indent (spaces) — matches the editor's default tab size (spec §07). */
+const DEFAULT_INDENT = 2;
+
+/**
+ * Reformat `text` as consistently-indented JSON, or return `null` when it is not
+ * valid JSON. Returning `null` (rather than throwing) lets callers skip a no-op
+ * rather than mangle an in-progress edit, mirroring auto-save's "skip when
+ * unparseable" rule (spec §03B). The result is idempotent: formatting
+ * already-formatted text returns it unchanged.
+ */
+export function formatJson(
+  text: string,
+  opts: { indent?: number; maxLength?: number } = {},
+): string | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(text);
+  } catch {
+    return null;
+  }
+  return stringify(parsed, {
+    indent: opts.indent ?? DEFAULT_INDENT,
+    maxLength: opts.maxLength ?? DEFAULT_MAX_LINE,
+  });
+}