The client had no test infra. Add bun test + happy-dom + Testing Library (chosen over Vitest because the Nix checks sandbox is bun-only, so bun test is the only runner actually enforced by nix flake check, via a new client-tests derivation). Documents a division of responsibility (TESTING.md + a directory-scoped CLAUDE.md that auto-loads): unit tests (client/src/**/*.test.tsx, colocated) are the coding agent's to write and maintain; behavioral tests (client/tests/behavior/) are developer-owned and must not be added or changed by the agent without an explicit request (it may suggest one). Adds the first behavioral test: the session-rename pane closes when switching to a different tab. Setup notes: tests/happydom.ts is a separate first preload that registers the DOM before tests/setup.ts imports Testing Library (whose screen binds to document at import time); useStore's Node import is now type-only so the store imports without pulling @xyflow/react at runtime; test files are excluded from the shippable tsconfig; bun.nix regenerated for the new devDeps. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>