[Feature] usePopover: ship a first-party floating-ui adapter

[johnleider]

Follow-up to #846 (refs #733). The adapter seam lands there with CSS anchor positioning as the default; a first-party floating-ui adapter should ship from v0 itself rather than being left to consumers.

Shape (per the flagsmith/launchdarkly/posthog precedent in features/adapters/*):

• Subpath export (e.g. @vuetify/v0/popover/adapters/floating-ui) with @floating-ui/dom as an optional peerDependency
• computePosition + autoUpdate, default offset/flip/shift middleware, constructor accepts middleware overrides
• positionTry documented as ignored under this adapter (flip() covers the intent)
• Unit tests (mocked) + one browser test asserting real coordinates
• Docs: flip the use-popover FAQ to point at the shipped adapter; keep the bring-your-own sketch as an appendix

Lands on dev after #846 merges.

[johnleider]

Landed in #917 (merged to dev; GitHub won't auto-close from a non-default base).