docs: add docs folder (architecture, developer, usage); update README; wire probe TTL cache + concurrency notes into docs

docs/developer/testing.md

@@ -0,0 +1,27 @@
+# Testing
+
+## Unit and Integration
+```bash
+cargo test --workspace
+```
+
+## Gated Tests (require external services)
+- Runner NATS:
+```bash
+RUNNER_TEST_NATS_URL=nats://127.0.0.1:4222 cargo test -p runner -- --ignored
+```
+- Projection NATS:
+```bash
+PROJECTION_TEST_NATS_URL=nats://127.0.0.1:4222 cargo test -p projection -- --ignored
+```
+- Docker-based gates:
+```bash
+cargo test -p gateway -- --ignored
+```
+
+## Control UI
+```bash
+cd control/ui
+npm ci
+npm run test
+```