fix:docs

CONTRIBUTING.md

@@ -27,6 +27,25 @@ through Pinokio; the terminal path above remains the contributor path.
 
 Open `http://localhost:4173`. Before sending a PR run `npm run build`, `npm test`, and `npm run test:track` (dev server must be up) — **all three must stay green.**
 
+### Test prerequisites
+
+- **`npm test`** — unit tests. Runs standalone; no dev server or credentials
+  required. Exit code is the gate.
+- **`npm run test:track`** — the 3D flight-tracking regression harness. It
+  drives the **real app** in headless Chromium, so it **requires a running
+  dev server** on the default port (`http://localhost:4173`). Pass
+  `--url <app-url>` to point it at another instance (e.g.
+  `node scripts/track-regression.mjs --url http://localhost:4272`). It does not
+  depend on live OpenSky / adsb.lol / AISStream (those are optional or
+  rate-limited locally); it installs a persistent `fetch` shim that returns
+  synthetic aircraft in the exact upstream payload shapes the layers parse.
+- **`npm run build`** — production Vite build. No server or credentials
+  required; it validates that the client bundle compiles cleanly.
+
+If you change runtime behavior, update `docs/CURRENT-STATE.md` and
+`CHANGELOG.md` in the same PR. For a deeper catalog of what each harness
+checks and what it needs to run, see [TESTING.md](TESTING.md).
+
 ## Good first contributions
 
 The highest-leverage places to jump in: