empty state vs. populated, zoom controls visibility tied to node count,
URL ?focus= preselection (matching id selects, missing id does not),
zoom-out clamping safety. $app/state mocked at module boundary so the
test can drive page.url and page.data.canWrite without a SvelteKit
runtime.
Six tests focused on user-observable behaviour — one logical behaviour
per test (Sara's guidance).
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
83ca262b75