Improve extensibility with plugin architecture.

[koculu]

Plugins for PureStack sites

Adds a plugin system and loads it from the CLI, so a site extends PureStack without a build script of its own.

What's new

• definePlugin: a named bundle of skins, components, templates, markdown (remark/rehype plugins), pages (generated pages), hooks, and devMiddleware. Plugins are validated when a build starts, and errors name the plugin.
• purestack.config.ts: lives next to siteConfig.json and is loaded by build, serve, and publish. serve reloads it when the config or a local file it imports changes. Packages resolve to the copies the CLI runs.
• Generated pages: pages() returns { path, source } entries that build like files: routes, navigation, links, partials. source is a string, or a function PureStack calls only when it needs the content, so the plugin controls memory and caching. In serve, pages regenerate when content or assets change.
• onPageDocument hook: changes a page's DOM before it becomes HTML, and may be async. Each render gets its own document through an injectable async scope (useDomScope in ts-minidom). ts-ssg installs AsyncLocalStorage, so overlapping renders never see each other's pages. As a result, renderApp is now async.
• Dev middleware: plugins can answer requests during serve.

Fixes and cleanup

• A full build now prepares content once. Discovery, generators, navigation, and partials used to run twice, once at builder creation and again in buildAll.
• Removed the global component registry mutation from renderApp, along with createDom cleanup clearing whatever document happened to be current.
• The Studio site runs on plain purestack serve/build/publish. frontend/studio.ts is removed; studioPlugin.ts and purestack.config.ts replace it.

Docs

New Plugins guide. Updated the CLI guide, the READMEs (ts-ssg, purestack, ts-render, ts-minidom), and the incremental build notes.

Verification

• All tests pass, including new tests for plugins, config loading, generated pages, source functions, overlapping renders, and single preparation.
• TS7 type-check and biome are clean.
• The Studio build output is unchanged.