feat(react): add the AlertDialog primitive

[ivanbanov]

Summary

Adds the AlertDialog primitive as two packages:

• @dunky.dev/alert-dialog — the framework-free state machine (core), built on @dunky.dev/state-machine.
• @dunky.dev/react-alert-dialog — the thin React binding.

An alert dialog is a modal window that interrupts the user with an urgent question and requires a response, per the WAI-ARIA APG alertdialog pattern. Where a plain dialog can be waved away, an alert dialog cannot: it is always modal, an outside press never dismisses it, and the only ways out are the Cancel/Action parts and Escape.

Stacked PR: this branch is based on feat/floating (the shared overlay utilities extraction). It targets feat/floating for now and will retarget to main when that PR merges.

Anatomy

<AlertDialog>          - root; owns open/close state, renders nothing of its own
  |_ <Trigger>         - opens the alert dialog; focus returns here on close
  |_ <Portal>          - moves the layer to the document body
     |_ <Backdrop>     - the layer behind the window; never dismisses
     |_ <Viewport>     - positioning and scroll layer; never dismisses
        |_ <Content>       - the alert dialog window itself
           |_ <Title>       - names the alert dialog
           |_ <Description> - states the situation that demands a response
           |_ <Cancel>      - the least destructive answer; closes; takes initial focus
           |_ <Action>      - the confirming answer; closes

Behavior

• Always modal: role="alertdialog" + aria-modal="true", focus is trapped inside, background scroll is locked, and outside presses never dismiss.
• Initial focus lands on Cancel — the least destructive answer; focus returns to the trigger on close.
• Escape closes, vetoable via onEscapeKeyDown (event.preventDefault()); backdrop/viewport presses are not a dismissal.
• Controlled open is truly controlled: every open/close intent (trigger press, Escape, Cancel/Action) is reported through onOpenChange, but the alert dialog only moves when the prop does — ignoring a report is a working veto (e.g. hold it open while an async destructive operation resolves). Prop-driven transitions are not echoed back.
• aria-labelledby/aria-describedby follow the rendered Title/Description — an omitted part never leaves a dangling reference.
• data-state="open|closed" on the stateful parts is the styling hook; no positioning engine — the Viewport is the consumer's layout layer.

Test plan

• pnpm test:ci — 145 tests, 13 files, all passing
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

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