feat(react): add the Popover primitive

[ivanbanov]

Summary

Adds the Popover primitive across both scopes:

• @dunky.dev/popover (packages/core/popover) — the framework-free state machine and connect layer.
• @dunky.dev/react-popover (packages/react/popover) — the thin React compound-component binding, plus stories and a changeset.

A popover is a non-modal floating panel opened from a trigger that coexists with the page: Escape, an outside press, or focus leaving the panel dismisses it, and the modal option opts into a focus trap with the outside hidden from assistive tech instead. Open state is exposed controlled and uncontrolled; with the open prop set, every open/close intent is reported through onOpenChange and the popover only moves when the prop does, so ignoring a report is a working veto. No positioning engine in v0 — the consumer anchors the panel, and data-state on the trigger and panel is the styling hook.

Stacked PR: this branch is based on the feat/floating shared-overlay-utilities extraction and targets that branch; it will retarget to main once the extraction PR merges.

Anatomy

<Popover>              — root; owns open/close state, renders nothing
  |_ <Popover.Trigger>     — toggles the popover; focus returns here on close
  |_ <Popover.Portal>      — teleports the panel out of the tree while open
     |_ <Popover.Content>      — the floating panel (role="dialog")
        |_ <Popover.Title>       — the popover's accessible name
        |_ <Popover.Description> — the popover's accessible description
        |_ <Popover.Close>       — the visible in-panel close affordance

Behavior

• Trigger carries aria-haspopup="dialog", aria-expanded, and — while open — aria-controls; a press while open is a toggle, never an outside interaction (no dismiss-then-reopen flicker).
• Content is role="dialog", aria-modal only when modal; labelled/described by the rendered Title/Description with no dangling references when a part is omitted.
• Focus on open goes to the consumer-designated initial-focus element, else the first focusable in the panel, else the panel itself; on close it returns to the previously focused element — except after a focus-out dismissal, where it stays where the user sent it.
• Non-modal by default: the page stays interactive, Tab flows out of the panel, and focus leaving counts as an outside interaction. modal traps focus and hides everything outside from assistive tech.
• Escape and outside-interaction dismissal are each gateable at the root and vetoable per occurrence from their handlers.
• Participates in the shared overlay layer stack: nested overlays stack, Escape unwinds one layer per press, and interactions inside a nested layer never count as outside the one beneath.

Test plan

• pnpm test:ci — 158 tests, 13 files, all passing
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

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