Cover the app entry with tests

[BJMCox]

src/main.ts had no tests. It is where every bug found by hand-driving the built app actually lived — the failed load that stripped ?sample=&node= from a model still on screen, the onerror handler that Chrome never fires, the search stepping that hung off change — and none of those were visible to the suite. CI now runs on every push, so it was worth giving it something to protect.

The harness

Importing main.ts wires the whole UI to the current document, so it cannot be called into. test/app-harness.ts rebuilds the DOM and re-imports the module for each case (vi.resetModules() + dynamic import). The DOM is read from the real index.html, not restated, so a control main.ts looks up by id but the markup has lost fails loudly instead of silently disabling a feature.

Fakes for what jsdom lacks, each documented in the harness:

Fake	Why
Worker	jsdom has none. Records what main.ts posts and answers on demand — answer() runs the real detectAndParse, i.e. the worker's own body — so the ready handshake, load tokens, superseded replies and the startup watchdog are all drivable.
fetch	Bundled samples read off disk from public/.
localStorage	Undefined under this runner; main.ts optional-chains it, so the persistence paths would otherwise just be skipped.
<dialog>	jsdom implements only open — no showModal, close, returnValue. submitDialog() stands in for the browser's form[method=dialog] step, so these cases cover our handlers, not the browser semantics under them.
Element.scrollIntoView	No layout in jsdom, and the tree scrolls the selected row into view on every selection.

Coverage

67 cases, 180 → 247 in the suite.

Loading (main-load.test.ts): startup empty state and the one-click sample CTA; the registry-derived dropdown; sample fetch, parse and reporting; fetch failure; the file picker; drag-and-drop including the overlay's enter/leave/child-crossing behaviour; paste, cancel and empty paste; the large-file warning both ways; parse failure keeping the previous model; superseded replies ignored; the watchdog firing for a worker that never starts and not firing for a slow parse once ready; onerror and onmessageerror; the deep-linkable URL, including an unknown sample and an unknown node.

Interaction (main-ui.test.ts): search by id, by type, no-match, empty; Enter stepping and wrapping; change staying idempotent; debounced suggestions; / and Escape, including that typing is never hijacked; pane resizing by keyboard and by pointer, clamping, Home, and persistence; pane tabs; the model dialog's summary, metadata, worst-first diagnostics, node xref, and the diagnostic count button.

Verification

The suite passes 247/247, three consecutive runs, plus tsc --noEmit and the build.

The two cases carrying regression notes were mutation-checked: reverting each fix in main.ts makes exactly that case fail (and reverting the search fix fails 7 search cases). The watchdog case was checked the same way. So these aren't tests that merely pass.