docs: browser runtime design (craft-libs, workers, #1221) (#1258)

A design note for SolidCraft in the browser: tiered storage, a SharedWorker job broker, GPU dispatch and in-browser agent control through the same engine commands, with the generic pieces in craft-libs. Linked from the MCP and web getting-started docs.

docs/book/src/guide/getting-started.md

@@ -74,7 +74,9 @@ doesn't update automatically.
 
 The same app runs in the browser: [open SolidCraft on the web](../app/) (`?sample` opens the sample
 part, `?webgl` forces WebGL2 where WebGPU is missing). Designs are saved in the browser's private
-file storage, which needs HTTPS or localhost.
+file storage, which needs HTTPS or localhost. Planned worker and tiered-storage integration is
+described in [web-runtime.md](https://github.com/bherbruck/solidcraft/blob/main/docs/web-runtime.md)
+(design; not yet wired in the shipped wasm build).
 
 After the first visit the web app works offline: the browser keeps a copy of it. Online, each
 launch gets the latest published build (the browser downloads it once when it changes); offline,