feat(createStack): add overlay z-index management composable

[johnleider]

Summary

• Add createStack composable for managing overlay z-index stacking with automatic calculation and scrim integration
• Add Scrim component for shared overlay backdrops using the stack context
• Integrate useStack into DialogContent for proper z-index coordination when multiple dialogs are open

Features

createStack Composable

• Global reactive registry of active overlays built on createRegistry
• Automatic z-index calculation (baseZIndex + increment per overlay)
• globalTop/localTop tracking for keyboard event handling (e.g., Escape key)
• Parent/child tracking via injection for nested overlays
• SSR-safe with per-app isolation via createStackPlugin
• Stack context with dismiss(), scrimZIndex, and isBlocking support

Scrim Component

• Uses stack context for automatic z-index positioning below top overlay
• Supports teleport to body and configurable transitions
• Handles blocking state for non-dismissable dialogs

Dialog Integration

• DialogContent now registers with stack for z-index coordination
• Added blocking and disableGlobalStack props
• Exposes globalTop and zIndex in slot props

API

// Basic usage
const isOpen = shallowRef(false)
const { styles, globalTop, zIndex } = useStack(isOpen, {
  baseZIndex: 2000,
  onDismiss: () => { isOpen.value = false }
})

// Stack context for scrim
import { stack } from '@vuetify/v0'
stack.isActive.value    // boolean - any overlays active?
stack.scrimZIndex.value // number - z-index for scrim
stack.dismiss()         // dismiss top non-blocking overlay

Test plan

• Unit tests pass for createStack composable
• Dialog tests pass with stack integration
• Type checking passes
• Linting passes for changed files

[pkg-pr-new]

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commit: b098e42