refactor(eslint): ban async vi.mock factories with dynamic import in body

Generalise the no-restricted-syntax rule from the literal pdfjs-dist
selector (added in #535) to also catch the underlying mechanism named in
ADR-012 / #553: any `vi.mock(..., async () => { ... await import(...)
... })` produces a late birpc roundtrip during worker teardown.

Selector: vi.mock CallExpression whose second argument is an
ArrowFunctionExpression with async=true and whose subtree contains an
AwaitExpression > ImportExpression. Both rules coexist — the literal
pdfjs-dist rule still enforces the libLoader prop injection pattern
(catches sync forms too); the new rule enforces the sync-factory
invariant universally.

Demonstrated by planting a synthetic offender locally and watching
ESLint flag it with the new rule's message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/eslint.config.js

@@ -82,6 +82,17 @@ export default defineConfig(
 						"CallExpression[callee.object.name='vi'][callee.property.name='mock'] > Literal[value=/^pdfjs-dist/]",
 					message:
 						"Banned: vi.mock('pdfjs-dist', factory) causes a birpc teardown race in browser-mode specs — see ADR 012. Use the libLoader prop injection pattern instead."
+				},
+				{
+					// ADR 012 / #553. The named mechanism: an async vi.mock factory whose
+					// body performs `await import(...)` produces a late birpc roundtrip
+					// during worker teardown. The factory body must be synchronous; if
+					// you need to share state between the spec and the mock, use
+					// `vi.hoisted` (see DropZone.svelte.spec.ts).
+					selector:
+						"CallExpression[callee.object.name='vi'][callee.property.name='mock'][arguments.1.type='ArrowFunctionExpression'][arguments.1.async=true]:has(AwaitExpression > ImportExpression)",
+					message:
+						'Banned: vi.mock(..., async () => { await import(...) }) causes a birpc teardown race in browser-mode specs — see ADR 012. Use a synchronous factory + vi.hoisted instead.'
 				}
 			]
 		}