feat(solid-router): loadFlightTarget, the cache-agnostic single-flight trigger

We updated server-functions-handler.ts (source and pre-built dist) to access registerFlightDataSource via Object.assign({}, solidServerFunctions) rather than directly on the namespace binding. This prevents rspack's static ESM linking validation from rejecting the access against the installed @solidjs/web rc.4 exports list, where registerFlightDataSource has not yet shipped. Runtime behaviour is unchanged — the result is the function on rc.5+ or undefined on rc.4, and the existing hasNamedFlightSources guard handles both paths.