Use colocated public directories for browser assets

[brookslybrand]

This stacks on #11480 and explores using colocated public/ directories as the explicit boundary for browser-reachable app source. Features can keep their browser module graph beside its narrowest owner without recreating the app structure in a top-level assets directory or renaming every file.

• Allow app/**/public/** through the asset server, with app/routes.ts as the shared route-contract exception.
• Move the browser runtime to app/public/entry.ts and the starter's interactive prompt button to app/ui/public/prompt-button.tsx.
• Reserve root public/static/ for images, fonts, and other files served unchanged at root URLs.
• Generate the entry script URL with assetServer.getHref() so it follows asset-server routing and fingerprinting.
• Update the scaffold guide, Remix app skill, asset/UI examples, release note, and scaffold assertions to teach the same transitive dependency rule.

// Before
allowFiles: ['app/assets/**']
staticFiles('./public', { index: false })

// After
allowFiles: ['app/routes.ts', 'app/**/public/**']
staticFiles('./public/static', { index: false })

Compiled browser source can now be organized by owner:

app/
  public/
    entry.ts
  actions/cart/
    controller.tsx
    public/
      cart-button.tsx
      format-price.ts

[github-actions]

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