feat(cli): tsup -> tsdown -> Vite+ migration

[KTrain5169]

This PR adds a new step in the vp migrate flow: migrate tsup.config.ts configurations into Vite+ by using the tsdown-migrate tool.
Resolves #2101

Blockers

I'm currently blocked by tsdown-migrate requiring TTY and immediately crashing without it, so for now this PR will remain drafted. Once a new version of tsdown-migrate with support for non-TTY terminals has landed, I'll continue working on this PR. pinning to RC for now but I'll edit it later to unpin from RC

[netlify]

✅ Deploy Preview for viteplus-preview ready!

Name	Link
🔨 Latest commit	2c85799
🔍 Latest deploy log	https://app.netlify.com/projects/viteplus-preview/deploys/6a7d67e9b96ea300088d3226
😎 Deploy Preview	https://deploy-preview-2434--viteplus-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[github-actions]

Registry bridge build (2c85799)

Warning

This build is from the fork KTrain5169/vite-plus and has not been reviewed.
Installing it runs that code on your machine. Build log

This commit build is published to the registry bridge, which serves these as ordinary npm versions (every other package proxies to npmjs):

Package	Version
vite-plus	0.0.0-commit.2c857991cf429f3636b5ac65acfe2eff2deed30e
@voidzero-dev/vite-plus-core	0.0.0-commit.2c857991cf429f3636b5ac65acfe2eff2deed30e

Install the Vite+ CLI built from this commit, then migrate a project:

# macOS / Linux
curl -fsSL https://vite.plus | VP_PR_VERSION=2434 bash

# Windows (PowerShell)
$env:VP_PR_VERSION="2434"; irm https://vite.plus/ps1 | iex

After installing, upgrade the current project's vite-plus to this test build with:

vp migrate

Or point your package manager at the bridge registry https://registry-bridge.viteplus.dev/:

Package manager	Registry config
npm / pnpm / Bun	.npmrc: registry=https://registry-bridge.viteplus.dev/
Yarn (v2+)	.yarnrc.yml: npmRegistryServer: "https://registry-bridge.viteplus.dev/"

Then pin the build (vite aliases to vite-plus-core; pnpm can use a catalog, npm an overrides entry):

{
  "devDependencies": {
    "vite-plus": "0.0.0-commit.2c857991cf429f3636b5ac65acfe2eff2deed30e",
    "vite": "npm:@voidzero-dev/vite-plus-core@0.0.0-commit.2c857991cf429f3636b5ac65acfe2eff2deed30e"
  }
}