Add host-extension slot seam for wrapping deployments

[benvinegar]

What

Adds a small, generic seam so a wrapping deployment (e.g. a hosted SaaS) can render its own UI inside the viewer chrome without forking the viewer — and without shipping a second copy of Solid (the single-runtime rule). Self-hosted deployments register no slots and behave exactly as before.

How

• Runtime global — the viewer publishes its own Solid instance as window.__SIDESHOW_SOLID__ ({ ...solid-js, web, store }), the stable contract a host bundle compiles against so it shares the viewer's runtime instead of bundling its own.
• Reactive slot registry — window.sideshow.registerSlot(name, Component) stores registrations in a createStore, so late/async registration after boot still renders. Each named slot renders via <Dynamic> inside the viewer's root owner, so theme/context/signals are shared. One slot is wired today: account (topbar right).
• Shared context — slot components receive a small HostContext (theme signal + setter, theme list, active session id, base path).
• Readiness event — sideshow:ready is dispatched after the runtime is published and the app is rendered, so a deferred host bundle knows when to register.
• headHtml AppOption — generalizes the existing __SIDESHOW_BASE_PATH__ head injection so a host can load a companion bundle / set globals. Documented as deployment-controlled (inserted verbatim).

All inert for self-hosters: no slots registered → the global is an unused handle and the chrome is unchanged.

Notes

• viewer/src/host.tsx is the whole contract; App.tsx adds <Slot name="account"/>; main.tsx calls announceHostReady() post-render; server/app.ts gains the optional headHtml.
• Changeset included (minor).
• Reviewed by multiple independent passes; viewer typechecks + builds with the seam present in the bundle.

🤖 Generated with Claude Code