Hold the runtime's NATS client to the shared fixtures

Read back from the stream rather than from the client that wrote it, so the
check is what reached the wire. The runner lives with the implementation;
the fixture stays in one place.

test/conformance.mjs

@@ -0,0 +1,54 @@
+// The TypeScript implementation, held to the shared fixtures (novox/hq ADR 0074, design 19).
+//
+// Run against a NATS server, because the question is what actually reaches the wire:
+//
+//   docker run -d --rm --name c -p 14222:4222 nats:2.10-alpine -js
+//   node test/conformance.mjs
+//
+// **The runner lives with the implementation it exercises; the fixture does not.** It is read
+// from the sdk's conformance directory by sibling path — the same file the Go suite reads. A
+// fixture copied into each implementation is two fixtures, and two fixtures drift, which is the
+// failure the suite exists to prevent.
+import { readFileSync } from "node:fs";
+import { connect } from "nats";
+
+const clientPath = process.argv[2] ?? "../dist/broker-nats.js";
+const { connectNats } = await import(clientPath);
+const f = JSON.parse(readFileSync(new URL("../../mesh-sdk/conformance/events/module-event.json", import.meta.url)));
+const URL_ = process.env.MESH_TEST_NATS ?? "nats://127.0.0.1:14222";
+
+let failed = 0;
+const check = (ok, what) => { console.log(`  ${ok ? "ok  " : "FAIL"}  ${what}`); if (!ok) failed++; };
+
+const admin = await connect({ servers: URL_ });
+const jsm = await admin.jetstreamManager();
+await jsm.streams.add({ name: "EVENTS", subjects: ["mesh.mod.*.event.>", "mesh.seat.*.event.>"] });
+
+const shop = await connectNats({ url: URL_, node: f.given.node, module: f.given.module });
+await shop.publish({ key: f.given.key, node: f.given.node, body: f.given.body, headers: f.given.headers });
+
+// What actually landed, read back from the stream rather than from the client that wrote it.
+const msg = await jsm.streams.getMessage("EVENTS", { last_by_subj: f.wire.subject });
+check(!!msg, `it lands on ${f.wire.subject}`);
+
+if (msg) {
+  const got = {};
+  if (msg.header) for (const k of msg.header.keys()) got[k] = msg.header.get(k);
+  for (const h of f.wire.requiredHeaders) {
+    check(got[h] !== undefined && got[h] !== "", `${h} is set`);
+  }
+  check(got["content-type"] === f.wire.headerFormats["content-type"], "content-type is as pinned");
+  check(new RegExp(f.wire.headerFormats["x-event-id"]).test(got["x-event-id"]), "x-event-id is as pinned");
+  check(!Number.isNaN(Date.parse(got["x-time"])), "x-time parses as a date");
+  check(got["x-source"] === f.given.module, "x-source agrees with the subject's module");
+
+  const payload = JSON.parse(new TextDecoder().decode(msg.data));
+  check(payload.envelope === undefined && payload.key === undefined,
+    "the payload is the body alone, not the envelope (the fixture refuses nesting)");
+  check(JSON.stringify(payload) === JSON.stringify(f.given.body),
+    "the body round-trips — semantic, not byte-exact, per the README");
+}
+
+await shop.close(); await admin.close();
+console.log(failed ? `\n${failed} failed` : "\nall passed");
+process.exit(failed ? 1 : 0);