[Alcova] Implementation plan: renderBuiltUrl-aware asset URLs

[MarcPorciuncula]

Tracking PR for the Alcova implementation plan in `ALCOVA-PLAN.md`.

Do not merge. This PR exists to host review / discussion of the plan on the `alcova/plan` branch without landing the file on `main`. Keeping `main` free of Alcova-specific content is what makes future upstream PRs (from `alcova/render-built-url`) diff cleanly against `TanStack/router:main`.

What the plan covers

• Diagnosis: why the current operator-ui deploy breaks when Vite's `base` is set to a CDN URL, and why that's a `base` misuse but also an exposed gap in TanStack Start's interaction with `experimental.renderBuiltUrl`.
• Design: route manifest-builder asset URL emission through an injected `resolveBuiltUrl(fileName)` that consults `config.experimental.renderBuiltUrl` before falling back to `joinURL(basePath, fileName)`.
• Implementation scope: three files touched —
  • `packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts`
  • `packages/start-plugin-core/src/vite/start-manifest-plugin/plugin.ts`
  • `packages/start-plugin-core/src/vite/prerender.ts`
• Tests: unit tests on `createManifestAssetResolvers` for the new resolver and its fallbacks; prerender integration coverage for absolute-`base` preview servers.
• Local verification: step-by-step `pnpm link` into `operator-ui`, drop the existing `pnpm` patch, flip `vite.config.ts` to `base: "/"` + `renderBuiltUrl`, run the dual-origin smoke test (HTML on :3000, CDN on :3001, real backend on :8080).
• Upstream workflow: open the upstream PR from `Alcova-AI:alcova/render-built-url` → `TanStack/router:main`. Implementation branch stays Alcova-free; plan stays on `alcova/plan`.

The next agent's entry point is `ALCOVA-PLAN.md` in this diff.

[MarcPorciuncula]

Closing — solved upstream.

transformAssets (merged in TanStack/router#7009, available in @tanstack/react-start@1.167.4+) is the canonical solution for splitting HTML and assets across origins. Alcova-side migration: Alcova-AI/operator-ui#417. Closing companion PR #2 too; archiving the fork.