Brings the v2 rc line (through 2.0.0-rc.6) into the Solid 2 branch.

Ported main features onto the Solid 2 architecture:
- Server-function error handler (#2262): wired through
  handleServerFunctionRequest's transformResult hook and the
  solid-start:server-fn-error-handler virtual module.
- Custom seroval plugins (#2250): serialization.plugins now feeds the
  @solidjs/web codec on both the client transport and the server handler.
- ~ alias fix for workspace packages (#2239): adopted appRootAlias in
  place of the global resolve.alias entry.
- Dev toolbar (#2049, replaces the dev overlay): ported to Solid 2
  (Errored/Loading, onSettled, two-arg createEffect, draft-mutation
  stores, terracotta 2 subpath imports, @solidjs/web Portal/Dynamic/JSX,
  createMemo(async) instead of createResource). Protocol-specific pieces
  (SerovalChunkReader, body-format markers) moved local to the toolbar.
- Deferred stream response via h3 iterable() (#2231), StartHandler type
  (#2234), routerLoad restore (#2228), nonce on streaming redirect
  script (#2252), ResizeObserver-loop filter (#2240) all kept.

Kept deleted (superseded by vite-plugin-solid / @solidjs/web /
@solidjs/router server-function integration): fs-routes tree-shake,
manifest.ts, directives, fns serialization/registration/shared and their
specs. Fixes main landed in those files (#2249, #2238, #2243, #2245,
#2241, #2261) live upstream now and are tracked for porting there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>