feat(react): add the Menu primitive

[ivanbanov]

Summary

Adds the Menu primitive as two packages:

• @dunky.dev/menu (packages/core/menu) — the framework-free state machine: open/close (controlled and uncontrolled), item registry, single-value highlight, typeahead, and layer-stack-aware dismissal.
• @dunky.dev/react-menu (packages/react/menu) — the thin React binding: compound parts, portal rendering, focus management, and data-state styling hooks. No positioning engine; anchoring is the consumer's concern.

A menu is a list of actions opened from a button: the user opens it, picks one action, and the menu goes away. It is non-modal and coexists with the page; every item is a command.

Note

This PR is stacked on the feat/floating extraction PR (shared overlay utilities) and targets that branch; it will retarget to main once feat/floating merges.

Anatomy

<Menu>                 - root; owns open/close state and the highlight
  |_ <Menu.Trigger>    - the button that opens the menu; focus returns here on close
  |_ <Menu.Portal>     - renders the content into a portal target
     |_ <Menu.Content> - the menu surface; holds DOM focus while open
        |_ <Menu.Item>       - one action; activating reports the selection and closes
        |_ <Menu.Group>      - groups related items under a name
        |  |_ <Menu.GroupLabel> - names its group
        |_ <Menu.Separator>  - divides sets of items

Behavior

• WAI-ARIA menu-button pattern: trigger is a button with aria-haspopup="menu" + aria-expanded; content is role="menu" labelled by the trigger; items are role="menuitem"; groups/role="group" labelled by their rendered GroupLabel; separators are role="separator". aria-controls only references the content while it is rendered.
• Keyboard, on the trigger: Enter/Space/ArrowDown open with the first enabled item highlighted; ArrowUp opens with the last. A pointer press opens with nothing highlighted.
• Keyboard, while open: ArrowDown/ArrowUp move the highlight with wrap, Home/End jump to the ends, printable characters run typeahead (repeating a character cycles matches), Enter/Space activate the highlighted item, Escape closes and returns focus to the trigger, Tab closes and lets focus continue.
• Highlight via aria-activedescendant: DOM focus stays on the content; the highlight is machine state, not roving focus. Disabled items are perceivable (aria-disabled) but never highlighted or activated.
• Dismissal: Escape, Tab, and outside press/focus close the menu; Escape and outside interaction are veto-able through their callbacks, and every intent reports through onOpenChange. A controlled menu never self-transitions — the open prop drives it.
• Layering: registers as a non-modal layer on the shared overlay stack — Escape/outside dismissal belong to the topmost layer only.

Test plan

• pnpm test:ci — 13 files, 182 tests passing
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

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