fix(test): Hotfix #6 — polyfill ResizeObserver in vitest setup (Phase 1 Combobox)

CI surfaced an Unhandled Error after the full vitest suite ran clean:

  ReferenceError: ResizeObserver is not defined
    at p (node_modules/@headlessui/react/dist/utils/element-movement.js:1:332)
    at combobox-machine.js:1:8089
    at y.send (machine.js:1:1383)
    at Object.closeCombobox (combobox-machine.js:1:5820)
    ... originating from src/components/Combobox.test.tsx

Test Files  60 passed (60)
     Tests  654 passed (654)
    Errors  1 error                ← vitest exits 1 on unhandled

Diagnosis
=========
Headless UI's Combobox + Dialog use ResizeObserver internally to
track trigger-element position (focus-management edge cases on
scroll / resize). jsdom does not implement ResizeObserver — without
a polyfill, Headless UI's async cleanup fires *after* the vitest
test completes (during the keyboard-nav close path) and throws the
ReferenceError as an Unhandled Error. The test assertions had
already passed; the unhandled exception alone causes vitest's
process exit to flip to 1.

Locally the error appeared as a "1 error" line below the green
summary but exit was still 0 because we ran with a tight timeout
that masked the post-test cleanup. The amd64 CI runner with the
full ~40s budget triggers the unhandled handler and propagates the
non-zero exit.

Fix
===
web/src/test/setup.ts adds a minimal ResizeObserverStub class
(observe / unobserve / disconnect are no-ops) and assigns it to
globalThis.ResizeObserver iff undefined. The component never reads
the observed dimensions in our test paths — the read sites fire
only after layout has settled in a real browser — so a no-op
construct + observer trio is sufficient to silence Headless UI's
internal calls.

Also stubs Element.prototype.scrollIntoView (Headless UI touches
it during Combobox.Options keyboard nav; jsdom warns rather than
throws but the CI log stays cleaner).

Verification
============

  $ cd web && npx vitest run src/components/Combobox.test.tsx
    Test Files  1 passed (1)
         Tests  5 passed (5)
    (no Unhandled Errors line; exit 0 — the post-test cleanup
    no longer touches the undefined global)

  $ cd web && npx tsc --noEmit
    (exit 0)

This commit ships on top of Phase 1 (e37403ed). The 654-test
green-suite count is unchanged; only the post-suite cleanup
behaviour changes.

web/src/test/setup.ts

@@ -1 +1,32 @@
 import '@testing-library/jest-dom';
+
+// Phase 1 (Foundation Primitives) global polyfills
+// =================================================
+// Headless UI's Combobox + Dialog use ResizeObserver internally to
+// track trigger-element movement (focus-management edge cases on
+// scroll / resize). jsdom does not implement ResizeObserver — without
+// a polyfill, Combobox.closeCombobox's async cleanup fires after the
+// vitest test exits and throws "ReferenceError: ResizeObserver is not
+// defined" as an Unhandled Error. The test assertions pass; the
+// unhandled exception causes vitest to exit 1.
+//
+// A minimal stub is sufficient. The component never reads the
+// observed dimensions in our test paths (those code paths fire only
+// after layout has settled in a real browser); it just needs the
+// constructor + observe/unobserve/disconnect to exist as no-ops.
+class ResizeObserverStub {
+  observe(): void {}
+  unobserve(): void {}
+  disconnect(): void {}
+}
+if (typeof globalThis.ResizeObserver === 'undefined') {
+  globalThis.ResizeObserver = ResizeObserverStub as unknown as typeof ResizeObserver;
+}
+
+// Headless UI also touches Element.prototype.scrollIntoView during
+// keyboard navigation of Combobox.Options. jsdom emits a noisy
+// warning but doesn't throw — still cheaper to stub it so the
+// CI log stays clean.
+if (typeof Element !== 'undefined' && !Element.prototype.scrollIntoView) {
+  Element.prototype.scrollIntoView = function () {};
+}