docs: streamline README and split guides

docs/testing.md

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+# Testing
+
+## Go
+
+Targeted offline-ish packages:
+
+```bash
+go test ./internal/live ./internal/models ./internal/store ./internal/ingest ./internal/query ./internal/web
+```
+
+All packages:
+
+```bash
+go test ./...
+```
+
+OpenF1 integration tests require network access and are rate-limit aware:
+
+```bash
+go test -v ./internal/api
+```
+
+## Frontend Unit Tests and Build
+
+```bash
+npm --prefix frontend test -- --run
+npm --prefix frontend run build
+```
+
+## E2E
+
+The default Playwright config starts a seeded Go server on port `18080` and Vite on `15173`.
+
+```bash
+npx playwright install   # first time only
+npm run test:e2e
+```
+
+Production serving mode builds around Go serving `frontend/dist`:
+
+```bash
+npm run test:e2e:prod
+```
+
+## Visual Regression
+
+```bash
+npm run test:visual
+npm run test:visual:prod
+```
+
+After intentional UI changes:
+
+```bash
+npm run test:visual:update
+npm run test:visual:prod:update
+```
+
+Snapshots live under `tests/visual/__snapshots__/`.