Test older Vite versions

[yamcodes]

Right now we have a "vite-legacy" playground to softly back up the peer dependency range declared in @arkenv/vite-plugin:

"vite": "^2.9.18 || ^3.0.0 || ^4.0.0 || ^5.0.0 || ^6.0.0 || ^7.0.0 || ^8.0.0"

Current state

• vite-legacy is pinned to Vite 7 — it builds as part of pnpm run build via Turbo, so CI does verify at least one non-latest major.
• The Vite 2–6 range in peerDependencies is untested and unverified.

Long-term plan

Add a CI matrix job (in test.yml) that installs and builds vite-legacy against each claimed major:

strategy:
  matrix:
    vite-version: ["4", "5", "6", "7"]

Each job would override the vite dep (e.g. via pnpm --override vite@${{ matrix.vite-version }}) and run pnpm run build --filter=vite-legacy-playground.

Until that matrix exists, the peerDependencies floor should stay at ^7.0.0 (the lowest actually tested version) to avoid false promises to users on older Vite versions. Once the matrix is in place, the floor can be lowered only as far as the matrix validates.