fix(runtime): add multi-app support for global var __NUXT__

[nicolaspayot]

resolves #1521

This PR adds multi-app support for global variable win.__NUXT__.

These changes are related to the multi-app issue in nuxt: nuxt/nuxt#21635 and required for the following draft PR: nuxt/nuxt#27263.

[socket-security]

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Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	nuxt-nightly@​5.0.0-29761125.c6a7d2b6

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[pkg-pr-new]

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npm i https://pkg.pr.new/@nuxt/test-utils@858

npm i https://pkg.pr.new/vitest-environment-nuxt@858

commit: d84d63a

[coderabbitai]

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⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (3)

• examples/multi-app/nuxt.config.ts
• examples/multi-app/package.json
• examples/multi-app/vitest.config.ts

💤 Files with no reviewable changes (1)

• examples/multi-app/package.json

🚧 Files skipped from review as they are similar to previous changes (2)

• examples/multi-app/nuxt.config.ts
• examples/multi-app/vitest.config.ts

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📝 Walkthrough

Walkthrough

The change adds multi-app Nuxt runtime state handling through nuxtAppId and nuxtFuture environment options. It adds a multi-app example with a counter page, composable, Vitest configuration, and Nuxt and browser tests. It also adds application shells and corrects app.vue import aliases in the workspace example.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: multi-app support for the global NUXT variable.
Description check	✅ Passed	The description directly explains the multi-app support change and links it to the reported Nuxt test-utils issue.
Linked Issues check	✅ Passed	The changes pass Nuxt multi-app settings into the test environment, support app-scoped NUXT, and add multi-app Vitest coverage for issue #1521.
Out of Scope Changes check	✅ Passed	The example application, configuration, and tests are directly related to validating the multi-app runtime fix.

✨ Finishing Touches

🧪 Generate unit tests (beta)

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