feat: add TanStack Start framework adapter (#16139)

Adds `@payloadcms/tanstack-start` — the first non-Next.js framework
adapter for Payload's admin panel.

## Background

The framework adapter pattern already landed across four PRs:
- [Server adapter](#16753)
- [Router adapter](#16763)
- [View adapter](#16803)
- [Layout adapter](#16840)

This pushed every Next-specific concern (routing, request init, server
functions, HMR) behind typed contracts in `payload` and made
`@payloadcms/ui` framework-agnostic.

This PR is the payoff: a working adapter built entirely on that
abstraction, proving the admin panel renders on a non-Next stack with no
forks of the UI.

## Motivation

- **Prove the abstraction.** The adapter contracts are only as good as a
second implementation. TanStack Start exercises every seam — a different
renderer, a different server-function transport, a different build tool
— and validates that `@payloadcms/ui` carries no hidden Next.js
assumptions.
- **Meet users where they are.** Not every project is on Next.js.
Decoupling the admin panel opens Payload to the broader React SSR
ecosystem, with TanStack Start as the first proof point.
- **Keep one UI.** Both adapters render the same `@payloadcms/ui`
components and data fetchers. There is no TanStack fork of the admin
panel — only a thin adapter package that satisfies the contracts.

## How it differs from the Next.js adapter

Same UI, different plumbing behind the contracts:

| | Next.js | TanStack Start |
| ---------------- | -------------------------- |
--------------------------------- |
| Server rendering | RSC flight payloads | SSR + route loaders |
| Server functions | `'use server'` actions | `createServerFn` |
| Request init | `next/headers` | `@tanstack/react-start/server` |
| Build / HMR | Webpack / Turbopack | Vite |

## Integration touch points

Wiring Payload into a TanStack Start app is a handful of file routes,
similar to the Next.js app dir shipped since Payload v3:

```
app/
├── __root.tsx                      # root shell — withPayloadRoot swaps in the admin document on /admin
├── _frontend.tsx                   # your app's layout route
├── _frontend/                      # your app's routes
├── _payload.tsx                    # admin layout route — mounts Payload providers
└── _payload/
    ├── admin.index.tsx             # /admin
    ├── admin.$.tsx                 # /admin/* (splat)
    ├── api.$.ts                    # /api/* — Payload REST handlers
    └── server.functions.ts         # config + importMap injection; server functions
```

**Root shell** — This is the highest level touchpoint that affects your
app. In your root route file, add the `withPayloadRoot` shell component:

```tsx
// app/__root.tsx
import { withPayloadRoot } from "@payloadcms/tanstack-start/client";

export const Route = createRootRoute({
  shellComponent: withPayloadRoot(MarketingRoot),
});
```

For all other file contents, see the `app-tanstack` directory in the
monorepo (subject to change). Docs to be provided in the future.

## Status

Experimental. Ships as a new package alongside the Next.js adapter;
nothing in the existing Next path changes at runtime.

---------

Co-authored-by: Jake Fletcher <jacobsfletch@gmail.com>

package.json

@@ -80,6 +80,7 @@
     "dev:generate-types": "pnpm runts ./test/generateTypes.ts",
     "dev:postgres": "cross-env PAYLOAD_DATABASE=postgres pnpm runts ./test/dev.ts",
     "dev:prod": "cross-env NODE_OPTIONS=--no-deprecation tsx ./test/dev.ts --prod",
+    "dev:tanstack": "cross-env PAYLOAD_FRAMEWORK=tanstack-start NODE_OPTIONS=\"--no-deprecation --max-old-space-size=16384\" tsx ./test/dev.ts",
     "dev:vercel-postgres": "cross-env PAYLOAD_DATABASE=vercel-postgres pnpm runts ./test/dev.ts",
     "devsafe": "node ./scripts/delete-recursively.js '**/.next' && pnpm dev",
     "docker:clean": "node ./scripts/docker-clean.ts",
@@ -121,6 +122,8 @@
     "test:e2e:prod:ci:noturbo": "pnpm prepare-run-test-against-prod:ci && pnpm test:e2e:prod:run:noturbo",
     "test:e2e:prod:run": "pnpm runts ./test/runE2E.ts --prod",
     "test:e2e:prod:run:noturbo": "pnpm runts ./test/runE2E.ts --prod --no-turbo",
+    "test:e2e:tanstack": "cross-env PAYLOAD_FRAMEWORK=tanstack-start pnpm runts ./test/runE2E.ts",
+    "test:e2e:tanstack:headed": "cross-env PAYLOAD_FRAMEWORK=tanstack-start NODE_OPTIONS=\"--no-deprecation --no-experimental-strip-types\" NODE_NO_WARNINGS=1 DISABLE_LOGGING=true playwright test --headed",
     "test:eval": "node ./test/evals/cli.ts",
     "test:eval:report": "cross-env NODE_OPTIONS=\"--no-deprecation --no-experimental-strip-types\" NODE_NO_WARNINGS=1 vitest --run --project eval --reporter=default --reporter=html --outputFile.html=test/evals/eval-results/report.html",
     "test:int": "cross-env NODE_OPTIONS=\"--no-deprecation --no-experimental-strip-types\" NODE_NO_WARNINGS=1 DISABLE_LOGGING=true vitest --project int --no-cache",
@@ -199,6 +202,7 @@
     "tsx": "4.22.4",
     "turbo": "^2.5.4",
     "typescript": "catalog:",
+    "vite": "~7.3.2",
     "vitest": "4.1.6",
     "wrangler": "~4.61.1",
     "zod": "4.3.6"