fix(composables): stop shadowing Nuxt's built-in useLayout (#384)

[AndreyYolkin]

Closes #384.

Problem

Nuxt 4.5.0 shipped a built-in useLayout. This module also auto-imports Vuetify's unrelated useLayout, and our registration silently won the name, so on every Nuxt >= 4.5 project with default options the framework's own composable became unreachable.

The two are not interchangeable:

	Nuxt 4.5	Vuetify
returns	Readonly<ComputedRef<LayoutName>>	{ getLayoutItem, mainRect, mainStyles }
outside <VApp>	works	throws [Vuetify] Could not find injected layout

Nuxt's documented example calls useLayout() in app.vue, and <VApp> normally lives in a layout below it — so the documented Nuxt pattern crashed for our users.

Reproduced on nuxt 4.5.2 before the fix (nuxi prepare in apps/playground):

WARN [NUXT_B6002] useLayout is already auto-imported by Nuxt as a built-in,
     and overriding it will likely cause issues.
     fix: Rename useLayout in vuetify so it no longer collides with the built-in auto-import.
WARN Duplicated imports "useLayout", the one from "#app/composables/layout"
     has been ignored and "vuetify" is used

Fix

moduleOptions.prefixComposables gains a new value 'auto', which becomes the default:

value	behaviour
'auto' (new default)	prefix only composables whose name a Nuxt/Vue auto-import source already owns
true / false / string[]	unchanged

The invariant, which the implementation is written against:

'auto' prefixes exactly the set Nuxt would report as NUXT_B6002, and nothing beyond it.

Detection is dynamic, so there are no Nuxt or Vuetify version gates — on Nuxt < 4.5 the intersection is empty and nothing changes.

Why hooks instead of addImports

addImports needs the finished array at module-setup time, but the reserved names only exist once imports:sources fires on modules:done (measured: nuxt.options.imports.presets holds a single preset at our setup, and no useLayout). So we snapshot in imports:sources and push in imports:extend — which is literally what addImports does internally, one step later.

Scope

Only framework-owned sources count (from starting with #, or vue/vue-demi/vue-router). Third-party module presets are deliberately excluded, so the module's auto-import names never depend on which other modules a project installed. A probe of apps/playground (which has @nuxtjs/i18n) found 35 presets / 214 names, with useLayout the only intersection against our 11 composables.

Verification

nuxi prepare in apps/playground on nuxt 4.5.2, after:

	before	after
NUXT_B6002 + Duplicated imports	2 + 2	0
useLayout	import('vuetify')	nuxt/dist/app/composables/layout
useVLayout	—	import('vuetify')

• 24 new unit tests for the preset walking and the prefix decision; integration assertion on the basic fixture, checked to fail when prefixComposables: false.
• Full suite 19 files / 82 tests, lint clean, prepack green.
• A dev-only info line names the rename and how to override it; verified visible under nuxi dev and silent under prepare.

Upgrading

Projects that used Vuetify's useLayout should rename those call sites to useVLayout. TypeScript flags them, since Nuxt's composable returns a different type. Documented in the composables guide.