Add round-trip ping tester to demo/

demo/reducer-rtt-ts/.gitignore

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+node_modules/
+dist/
+.spacetimedb-token

demo/reducer-rtt-ts/LICENSE

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+../../licenses/apache2.txt

demo/reducer-rtt-ts/README.md

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+# Reducer RTT Demo (TypeScript)
+
+This demo measures reducer roundtrip latency by:
+1. Calling the `add` reducer.
+2. Waiting for the resulting reducer event (observed on the inserted row callback).
+3. Printing RTT in milliseconds.
+
+## Layout
+- `spacetimedb/`: server module
+- `src/main.ts`: Node.js RTT client
+
+## Run
+
+1. Start a local SpacetimeDB server (keep this running):
+
+```bash
+spacetime start
+```
+
+2. Publish the server module:
+
+```bash
+cd demo/reducer-rtt-ts/spacetimedb
+pnpm install
+spacetime publish --server local reducer-rtt-demo
+```
+
+3. Run the client:
+
+```bash
+cd demo/reducer-rtt-ts
+pnpm install
+SPACETIMEDB_DB_NAME=reducer-rtt-demo pnpm dev
+```
+
+Optional env vars:
+- `SPACETIMEDB_HOST` (default: `ws://localhost:3000`)
+- `SPACETIMEDB_DB_NAME` (default: `reducer-rtt-demo`)
+- `RTT_INTERVAL_MS` (default: `2000`)