feat: publish coauthoring-sidecar JSON Schema + rung-neutral validator (F5 SLICE-1, PUC-5, #14)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/validate-sidecar.mjs

@@ -0,0 +1,35 @@
+#!/usr/bin/env node
+// validate-sidecar — rung-neutral conformance check (spec §6.4, PUC-5).
+// usage:
+//   node scripts/validate-sidecar.mjs <sidecar.json>
+// exit 0 = valid, 1 = invalid (per-error JSON paths on stderr), 2 = usage/IO.
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import path from "node:path";
+import Ajv from "ajv/dist/2020.js";
+
+const here = path.dirname(fileURLToPath(import.meta.url));
+const schema = JSON.parse(readFileSync(path.join(here, "..", "schemas", "coauthoring-sidecar.schema.json"), "utf8"));
+
+const file = process.argv[2];
+if (!file) {
+  console.error("usage: validate-sidecar.mjs <sidecar.json>");
+  process.exit(2);
+}
+
+let data;
+try {
+  data = JSON.parse(readFileSync(file, "utf8"));
+} catch (err) {
+  console.error(`cannot read/parse ${file}: ${err.message}`);
+  process.exit(2);
+}
+
+const ajv = new Ajv({ allErrors: true });
+const validate = ajv.compile(schema);
+if (validate(data)) {
+  console.log(`OK ${file} (schemaVersion ${data.schemaVersion})`);
+  process.exit(0);
+}
+for (const e of validate.errors ?? []) console.error(`${e.instancePath || "/"} ${e.message}`);
+process.exit(1);