fix(popover): backport focus-leave dismissal to v2

[alexwarren]

Summary

Backports the popover focus-leave dismissal behavior from PR #2298 to the v2 branch.

• Adds Classic popover focusout handling so menu popovers close when keyboard focus leaves the reference/popover pair.
• Adds Svelte popover focus-leave handling, including controlled popover close requests and nested role="menu" detection.
• Adds v2 docs guidance for menu popovers and restores the PR fix(popover): Fix popover focus-leave dismissal #2298 changeset.

Validation

• npm run lint:ts -w packages/stacks-classic passed.
• npm exec -w packages/stacks-svelte -- eslint src/components/Popover/Popover.svelte src/components/Popover/PopoverContent.svelte src/components/Popover/PopoverReference.svelte src/components/Popover/Popover.test.ts passed.
• npm run build:dependencies -w packages/stacks-svelte passed with existing Less and bundle-size warnings.

Local validation blockers

• Classic web-test-runner starts but fails before assertions because existing unit test imports hit local dependency module export errors from aria-query/lz-string.
• Svelte web-test-runner fails before assertions because the local icon package does not export existing symbols such as IconShieldXSm.
• npm run lint -w packages/stacks-svelte is blocked by the same existing missing icon exports across unrelated Svelte components/stories.
• Root npx prettier could not parse the existing v2 docs HTML because of a pre-existing unmatched ; Svelte files were formatted via the workspace Prettier command instead.

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[changeset-bot]

🦋 Changeset detected

Latest commit: b2e6145

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@stackoverflow/stacks	Minor
@stackoverflow/stacks-svelte	Major

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[asblanco]

LGTM 🐐 🐐 📰