desktop(test): make fallback smoke validate React runtime

[Whiteks1]

Summary

Closes #431.

This PR keeps the existing legacy fallback smoke path intact and adds an explicit React fallback smoke path for the selectable React runtime introduced in #430.

What changed

• Added npm run smoke:react:fallback.
• Extended scripts/smoke.js with --renderer=legacy|react while preserving legacy as the default.
• Added a stable React renderer signal through window.__quantlab and a DOM marker for smoke readiness.
• Made smoke workbench checks distinguish legacy shell readiness from React shell readiness.
• Updated the happy-path smoke flow so React can re-open Runs and navigate to Compare after selecting runs.
• Allowed smoke fixtures under QUANTLAB_DESKTOP_OUTPUTS_ROOT to be read by the React runtime without weakening local path boundaries.
• Restored listDirectory to the bridge contract shape { entries, truncated }.

Scope boundary

This does not make React the default renderer, does not touch #432 or #412, does not remove research_ui, does not redesign smoke broadly, and does not change layout/brand/Launch behavior beyond smoke observation.

Validation

• npm run -s typecheck
• npm run -s build
• npm run -s smoke:fallback
• npm run -s smoke:react:fallback
• node --test tests/snapshot-status.test.mjs
• git diff --check

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