fix(with-tanstack-router): use per-request router for SSR

[nerdchanii]

Summary

Closes #268.

The solid-start-v2/with-tanstack-router template shared a single module-level router instance across all SSR requests. Under concurrency this leaks match state and loader data between requests — deterministically, once any route has an async loader. The shipped demo hides the leak only because its routes are static with no loaders.

This PR keeps SSR but makes the server-side router per-request.

Changes

• entry-server.tsx — routerLoad now calls createRouter() (the factory) once per request and stashes the instance on event.locals.router, instead of mutating the shared singleton.
• app.tsx — reads the per-request router back via getRequestEvent()?.locals.router, falling back to the singleton for CSR.
• router.tsx — unchanged. It already exports the createRouter() factory. The module-level singleton export (router) is retained as the CSR fallback, since the browser shares no state across requests.

Why a per-request router?

A router instance holds its match cache and loader results. On @solidjs/start v2 (Nitro/h3) concurrent requests run on one event loop, so two requests sharing one router can commit one another's data — including user-scoped loader data. This reproduces 100% of the time once a route has an async loader (reproducer in #268).

CSR is unaffected: each browser tab is an isolated runtime that never shares a router across requests, so the singleton is safe there and is kept as the fallback.

Why the server router never leaks to the client

getRequestEvent resolves to a no-op returning undefined in the client bundle (solid-js/web ships voidFn as getRequestEvent), so getRequestEvent()?.locals.router short-circuits to the singleton in the browser. The server's router object never crosses the boundary; only the current request's dehydrated state is serialized into the HTML via $_TSR, as before.

Verification

• @solidjs/start@2.0.0 (stable): GET / and GET /about return 200 in both dev and build && start.
• Each request now builds its own router with its own match cache, removing the shared mutable state that produced the leak in [Bug] with-tanstack-router template: SSR returns 500 on @solidjs/start v2, and a possible router-singlelet concern #268. I'll re-run the reproducer from [Bug] with-tanstack-router template: SSR returns 500 on @solidjs/start v2, and a possible router-singlelet concern #268 against this patch and post the before/after numbers.

Notes

• This addresses the structural concern from [Bug] with-tanstack-router template: SSR returns 500 on @solidjs/start v2, and a possible router-singlelet concern #268 (the 500 was already fixed upstream by createHandler re-accepting the routerLoad arg on stable).
• Open to the maintainers' preferred naming (createRouter vs getRouter) and to splitting/combining commits as desired.

[nerdchanii]

@birkskyum Please confirm the Pull Request.