chore: bump solid-vite + lit-vanilla-demo consumers to vite 8.1

The two consumers whose plugins already support Vite 8 (vite-plugin-solid
^2 lists ^8; lit-vanilla has no framework plugin). Bumps vite ^6 → ^8,
resolving to 8.1.0 via the existing override. Shrinks the repo's Vite
major sprawl (was 5/6/7/8).

Verified per surface: build clean, tsc --noEmit clean, and full e2e
green — solid-vite-demo 10/10, lit-vanilla-demo 14/14 (incl. HMR
dev-server specs).

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

Author: serpentblade

examples/consumers/lit-vanilla-demo/package.json

@@ -25,6 +25,6 @@
     "@types/node": "^20",
     "playwright": "^1",
     "typescript": "^5.6",
-    "vite": "^6"
+    "vite": "^8"
   }
 }

examples/consumers/solid-vite/package.json

@@ -24,7 +24,7 @@
     "playwright": "^1",
     "source-map-js": "^1.2.1",
     "typescript": "^5.6",
-    "vite": "^6",
+    "vite": "^8",
     "vite-plugin-solid": "^2.11.12"
   }
 }