fix: isolate demo bundle globals for injected demo script

[voidborne-d]

Summary

• wrap the built demo bundle in an outer IIFE after Vite emits it
• add a regression check that verifies the wrapper is present
• keep the fix in the demo build pipeline so injected CDN/bookmarklet usage does not leak helper symbols globally

Testing

• npm run build:demo
• npm run test:demo-bundle

Context

Fixes the injected demo script regression reported in #426, where helper symbols from the generated bundle could collide with page-level bindings and trigger errors like Identifier 'St' has already been declared.

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