feat(modal): imperative store API + transition/collapse animation fixes

[wangdicoder]

Summary

• Modal context refactor. Adds an imperative store API on top of the existing context — show(id, props) returns a promise resolved by hide(result), with new Modal.Register, Modal.useModalActions, Modal.useModalSelf, Modal.store, and a named createModalStore factory. The legacy Modal.useModal(id) per-id hook is unchanged.
• fix(transition): move onExited side effect out of a setState updater (uses a stateRef for race protection). Eliminates "Cannot update component X while rendering Y" warnings when afterClose dispatches across components.
• fix(collapse): always mount <CollapseTransition>; the first time a panel expands from a closed start now animates instead of snapping open.
• fix(collapse-transition): stash onHidden in a ref so the animation effect depends only on visible. Unrelated parent re-renders no longer interrupt the running open/close animation (e.g. clicking Delete in the Deletable demo while another panel is animating).
• fix(docs): slugifyLink now extracts text from arbitrary React-node heading children, so MDX headings with inline <code> no longer crash the docs renderer.
• New ContextRegister demo + EN/ZH docs covering the new Context API and a "Choosing a store" section that warns about the singleton-default footgun.

Release

@tiny-design/react: minor — adds new public API. Existing Modal.useModal(id) / <Modal.Provider> / Modal.open/Modal.confirm callers continue to work without changes.

Test plan

• pnpm --filter @tiny-design/react test (817 tests, 99 suites) green
• In docs site (pnpm dev):
  • Modal → "Context" demo: Open Confirm / Open Settings work; toolbar trigger does not re-render on open/close.
  • Modal → "Registered modals with awaitable result" demo: Delete shows the modal once (single overlay), Delete/Keep resolves the promise; no React "Cannot update … while rendering" warning in console.
  • Collapse → "Deletable" demo: toggling a header animates smoothly without snap-back; clicking Delete on one panel does not interrupt another panel's animation.
  • Collapse → any panel that starts closed: first expand plays the open animation.