feat: add the Collapsible primitive

[ivanbanov]

Summary

Adds the Collapsible primitive as two packages:

• @dunky.dev/collapsible — the framework-free core: a state machine (built on @dunky.dev/state-machine) with two states, open and closed, plus a connect that translates state into the agnostic bindings vocabulary.
• @dunky.dev/react-collapsible — the thin React binding rendering the compound parts.

A collapsible is a disclosure following the WAI-ARIA APG Disclosure (Show/Hide) pattern: a trigger that shows and hides an associated content region in place. State is exposed controlled (open, follow + report, matching the dialog's contract) and uncontrolled (defaultOpen), with every transition reported through onOpenChange. Includes a changeset (minor for both packages).

Anatomy

<Collapsible>       - root; owns open/close state, renders no DOM of its own
  |_ <Trigger>      - the button that toggles the content
  |_ <Content>      - the region that is shown or hidden

Behavior

• Trigger renders a native <button> carrying aria-expanded and a constant aria-controls reference to the content; Enter/Space activation is native button semantics.
• Content stays mounted while closed — hidden via the native hidden attribute plus aria-hidden — so the ARIA reference never dangles and data-state can drive open/close animations.
• Disabled is a machine guard on user toggling only: the trigger stays focusable and announces aria-disabled instead of dropping out of the tab order; programmatic and controlled open/close stay ungated.
• Every part carries data-state (open / closed) and data-disabled — the styling and animation hooks.

Test plan

• pnpm test:ci — 9 files, 101 tests passing
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

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