Writes Vitest + RTL tests covering the virtual list threshold (>50 rows),
fallback path (≤50), directory collapse/expand, tab-switch scroll
preservation, file-click callback, and root-level files.
Also aliases react/react-dom to the parent monorepo copies in
vitest.config.ts so all components share the same ReactSharedInternals
dispatcher — fixing the pre-existing null-dispatcher hook errors that
affected all web component tests in this git worktree.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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