fix(experiment-tag): keep anti-flicker overlay up during redirect navigation

[tyiuhc]

Summary

Fixes a redirect "flash" in cross-subdomain redirect experiments.

#238 (cross-subdomain redirect impression tracking) made the redirect path async: applyVariants / handleRedirect became async and handleRedirect now awaits setMarketingCookie, which resolves the registrable domain. location.replace() only queues navigation — JS keeps running and the browser keeps painting the source page until the destination commits. The anti-flicker teardown (removeAntiFlickerCss) ran inside that window and stripped the overlay (including a customer's #amp-exp-css snippet), flashing the original page mid-redirect.

The previously released version was not affected because its redirect path was synchronous (fire-and-forget cookie write, no await before location.replace()), so there was no gap for the overlay to be torn down. The flash is specific to the async redirect path introduced in #238.

Changes

Gate every anti-flicker teardown path on a new isRedirecting flag:

• experiment.ts — set isRedirecting = true before location.replace(); skip removeAntiFlickerCss() and further remote processing in start() while a redirect is in-flight.
• index.ts — the bootstrap .finally() and the preview/extension path skip removeAntiFlickerCss() when client.isRedirecting. The preview path returns the startClient promise so teardown flows through the single isRedirecting-gated finally.

Test plan

• experiment.test.ts green (36 tests); asserts isRedirecting is set on the redirect path so the bootstrap skips teardown.
• eslint + prettier clean (0 errors).
• Manual: trigger a cross-subdomain redirect experiment and confirm no flash of the source page during navigation (normal + preview/extension paths).

Notes

• Builds on the async redirect path from feat: support cross-subdomain redirect impression tracking #238; merge this before / together with that cross-subdomain redirect work shipping in a release — otherwise the flash surfaces for all customers once that async path ships.