Collapsible `+/-` callout syntax lost during Markdown migration

[LeCarbonator]

Before migrating to TanStack Markdown, the website made use of rehype-callouts. Since they're implementing Obsidian Markdown syntax, it allowed for collapsible callouts using either + or - right after the callout type:

> [!TIP]- Title
> Folded content. - means default collapsed, + means default expanded.

However, when TanStack Markdown was introduced, it changed how callouts are parsed. This now causes documentation sites using that feature to have broken callouts:

• Polyfill requirements callout in Form installation

[tannerlinsley]

@tannerlinsley I reproduced this against the dependency contract and confirmed it needs an upstream syntax/API decision rather than a tanstack.com-only parser workaround.

Evidence:

• tanstack.com currently installs @tanstack/markdown 0.0.11; parsing > [!TIP]- Title with calloutsExtension() produces a plain blockquote, matching the broken page.
• 0.0.13 is the latest published package and has the same callout regex, so a dependency upgrade alone does not fix it.
• CalloutNode only carries kind, title, and children; there is no collapsed/expanded state to preserve the - or + marker.
• Both the React and HTML renderers unconditionally emit a .markdown-alert <div>, so even accepting the syntax would not restore disclosure behavior without extending the AST and renderer contract.
• Duplicate searches for collapsible/foldable callouts found only this issue and no open PR.

Proposed direction: add optional fold state to CalloutNode, parse the Obsidian -/+ marker in @tanstack/markdown, render foldable callouts with native <details>/<summary> semantics, publish the package, then bump tanstack.com with focused parser and renderer coverage. Keeping ordinary callouts unchanged avoids a breaking change for existing consumers.

Decision needed: should @tanstack/markdown officially preserve the prior Obsidian foldable-callout contract, including native disclosure rendering, or should the site intentionally drop that syntax and update affected documentation sources?