chore: move VR harness to vite 8 (Rolldown)

Bumps the visual-regression harness vite ^6 → ^8.1.0 (Rollup → Rolldown),
with framework plugins to their vite-8 majors: plugin-react ^4→^6,
plugin-vue ^5→^6, vite-plugin-svelte ^5→^7. vite-plugin-solid and
@analogjs/vite-plugin-angular ~2.5.1 already declare ^8.

Adds resolve.dedupe(['react','react-dom']) — plugin-react 6 + Rolldown
no longer auto-dedupes React, which would otherwise crash the react cells
with a null dispatcher (mirrors the consumer demo configs).

Build wins: full 6-target build 56.4s → 22.5s (2.51×), all 6 targets
green incl Angular. PIXEL parity NOT yet verified — gated on the Linux
Docker Playwright run before this is pushed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSYH6VBAJwa7nYy4AksuNH

Author: serpentblade