fix: resolve dev server deadlock when starting embedded Storybook

[srcdev]

🔗 Linked issue

Refs #993 — fixes the dev-server deadlock that bricks the Nuxt app. (Deliberately not "Resolves": the residual /_nuxt EAGAIN proxy-target errors from that issue surface once the deadlock is gone; a follow-up PR adapting #994's devServer.url fix will address those.)

📚 Description

As offered in #993 (comment), this fixes the startup deadlock introduced in #981:

• @nuxtjs/storybook starts Storybook inside Nuxt's listen hook and awaits it, blocking the boot pipeline (Vite middleware never attaches).
• @storybook-vue/nuxt's loadNuxtViteConfig detects the already-running Nuxt via tryUseNuxt() and waits for its vite:configResolved event — which can only fire once boot proceeds.

Circular wait. With current nuxt/cli every nuxt dev with the module enabled hangs on all requests forever; on older versions it surfaced as the EAGAIN proxy errors reported in #993. Storybook's manager UI still binds its port (it starts before the preview builder hits the deadlocked promise), which disguised the boot hang as a proxy problem.

The fix:

• module: capture the resolved client Vite config during module setup — the only moment registration is guaranteed to precede the event — and share it with the framework package via Symbol.for('@storybook-vue/nuxt:vite-config-promise') on the shared Nuxt instance
• module: start Storybook from the listen hook without awaiting it (failures still surface via the logger)
• addon: loadNuxtViteConfig consumes the captured promise instead of registering a hook that may already have missed the event; the previous hook registration remains as a fallback for embedding without the module
• module: pass ci: true to buildDevStandalone so a busy port can never block the (quiet-mode) startup on an invisible interactive prompt

Tests:

• New Playwright coverage for the embedded path (nuxt dev with the module starting Storybook), which was previously untested — the suite only exercised standalone storybook dev. Asserts the Nuxt app responds and a story renders, across Chromium/Firefox/WebKit (15/15 with the existing tests).
• test/module-integration.spec.ts updated: it previously asserted the deadlocking await pattern as correct.
• The playground now loads the module from the built package rather than raw TS source — embedded startup hangs under jiti's TS transform even with this fix (dev-rig-only; users always get dist). I'll file that separately, along with an embedded-mode docs-index gap found during testing.

Verified against the reproduction from #993 (braedenfoster/storybook-test-nuxt-4) with packed tarballs: app responds immediately, embedded Storybook fully boots (➜ Storybook: http://localhost:6006/), stories render.

(investigated and fixed with Claude Fable 5 assistance)

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commit: 127301c

[srcdev]

Update: pushed two follow-up commits — a CI fix (the playground now loads the module from the built package, so the root prepare script stub-builds packages first) and a fix for a config-mutation bug found while verifying against the #993 reproduction (mergeConfig shares nested references, so the addon was writing optimizeDeps settings into the running app's live config).

Full transparency on scope: during that verification I also found a deeper, pre-existing issue in how embedded mode shares the app's live Vite plugin instances with Storybook's server — filed as #1072 with full forensics. It affects plugin-heavy apps once this PR makes embedded mode bootable. This PR remains a strict improvement (embedded mode is 100% deadlocked without it), and the new issue documents the remaining work.

(investigated with Claude Fable 5 assistance)