Site Launch: ship semi-gated flow as default after gating experiment

[bogiii]

Proposed Changes

• Conclude the calypso_standardized_site_launch_gating_202603_v1 experiment by shipping the semi_gated_site_launch variant as the default behavior for everyone.
• Add a shared useSiteLaunchGatingVariant() hook (client/lib/explat/site-launch-gating.ts) that currently hardcodes the semi-gated variant. It lives under calypso/lib/explat/ so the Dashboard's existing ESLint import exception covers it.
• Replace the per-call-site useExperiment() calls with the shared hook in:
  • client/layout/masterbar/masterbar-launch-button.tsx
  • client/hosting/performance/site-performance.tsx
  • client/dashboard/sites/site-launch-button/use-site-launch.tsx
  • client/my-sites/customer-home/cards/launchpad/pre-launch.tsx
• Keep all existing variant branches (ungated, control/immediate-launch) as scaffolding. They are unreachable while the hook hardcodes the semi-gated variant, but they compile, lint, and stay covered by tests, so the expected follow-up experiment only needs a one-line change in the hook.
• Update the omnibar launch button test to mock the shared hook (instead of useExperiment) so the ungated scaffolding path remains tested.

Why are these changes being made?

• The site launch gating experiment finished and the decision is to ship the semi-gated flow (redirect to /start/launch-site) as the default.
• A follow-up experiment is expected soon, so instead of removing the experiment branching, the assignment source is centralized in one hook. Starting the next experiment means swapping the hardcoded return value for a useExperiment() call in a single file, with every launch surface (masterbar, performance page, dashboard launch button, launchpad) enrolled automatically.
• Calling useExperiment() for a finished experiment would keep firing useless assignment requests; the hardcoded hook avoids that.

Testing Instructions

• On an unlaunched simple site, click "Launch site" in the masterbar: you should be redirected to /start/launch-site (semi-gated flow) instead of launching directly.
• Repeat from the Site Performance page launch CTA and from the My Home pre-launch launchpad's "Launch site" task.
• In the new Dashboard, verify the site launch button (site settings visibility, performance callout, interim omnibar) links to the /start/launch-site flow.
• Verify A4A dev sites still get their dedicated launch URL/modal.
• yarn typecheck-client, yarn eslint on the touched files, and yarn test-client client/dashboard/app/interim-omnibar/test/omnibar-launch-button.test.tsx all pass.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[matticbot]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-183358

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-183358&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-183358&env=a8c-for-agencies

Dashboard Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-183358&env=dashboard

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-10f2d14b6182a0e43ce0421426fd077025b9fc01

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-10f2d14b6182a0e43ce0421426fd077025b9fc01&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-10f2d14b6182a0e43ce0421426fd077025b9fc01&env=a8c-for-agencies

Dashboard Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-10f2d14b6182a0e43ce0421426fd077025b9fc01&env=dashboard

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• agents-manager
• blaze-dashboard
• help-center
• notifications
• odyssey-stats
• wpcom-block-editor
• wpcom-smart-dictation

To test WordPress.com changes, run install-plugin.sh $pluginSlug update/site-launch-gating-ship-semi-gated-default on your sandbox.