Split demo site into its own package (#1351)

* Split demo site into its own package

Move the demo site from packages/webamp/demo/ into packages/webamp-demo/
as a standalone workspace package. This cleanly separates the library
(published to npm) from the demo site (deployed to webamp.org).

Key changes:
- Demo imports webamp source via relative paths (no aliases needed)
- Demo has its own package.json with demo-specific deps
- Simplified vite config (just nodePolyfills plugin)
- Removed vite branching from rollupPlugins.mjs (library-only now)
- Renamed build-library -> build in webamp package
- Updated turbo.json, CI, netlify config, and code-size workflow
- Removed demo-only deps from webamp package (sentry, butterchurn stays
  for the butterchurn bundle)
- Zero bundle size change for the library

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Fix: add type-check dependency to webamp#build task

The webamp-docs package needs type declarations from webamp to
type-check. These are generated by tsc (type-check), which must run
before the build task.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Add updated readme to webamp-demo package

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Document the live reloading dev flow in both readmes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Fix code-size CI: use deploy script that exists on both branches

The compressed-size-action checks out master and runs the build script.
Master doesn't have a root "build" script, so revert to using "deploy"
which exists on both branches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
         run: pnpm install --frozen-lockfile
       - name: Build all packages
         run: |
-          npx turbo build build-library
+          npx turbo build
         env:
           NODE_ENV: production
       - name: Lint and type-check