🍕 Fix buffer dependency stub

[jjpaulino]

What does this PR do?

Hardens the Vite buffer browser-compat stub so third-party libraries that subclass Buffer no longer crash the kiln-edit bundle at module-evaluation time.

The stub's fallback (used when the page has no global Buffer polyfill) was a plain object. Any dependency doing SafeBuffer.prototype = Object.create(Buffer.prototype) therefore hit Object.create(undefined) and threw — before Kiln could boot:

Uncaught TypeError: Object prototype may only be an Object or null: undefined
    at Object.create (<anonymous>)
    at vite-kiln-edit-init-…js

No linked issue — surfaced during the Vite kiln-edit rollout (context below).

Why are we doing this? Any context or related work?

Downstream in nymag/sites, edit mode began crashing the moment a component pulled crypto-browserify into the kiln bundle. Its transitive stack (create-hash, create-hmac, sha.js, cipher-base, …) depends on safe-buffer@5.2.1, whose module init runs SafeBuffer.prototype = Object.create(Buffer.prototype).

safe-buffer feature-detects from / alloc / allocUnsafe / allocUnsafeSlow; when all four exist it just re-exports the provided buffer module and skips subclassing. The old stub was missing allocUnsafe / allocUnsafeSlow and had no prototype (it was a plain object), so it fell into the subclassing branch and blew up.

The rewrite makes the fallback:

• a function with a real .prototype (subclass-safe),
• expose the full from / alloc / allocUnsafe / allocUnsafeSlow / concat surface, so safe-buffer takes the cheap re-export path, and
• add SlowBuffer for API parity.

A real globalThis.Buffer polyfill is still preferred when the page provides one.

Where should a reviewer start?

• The fix — BUFFER_STUB in browser-compat.js: function-shaped Buffer + full alloc API + SlowBuffer. The block comment documents the two constraints (real prototype; expose all four alloc methods).
• Tests — buffer specs plus the evalEsmStub sandbox helper: full alloc API present, fallback is subclass-safe when there is no global Buffer, and a real global Buffer is preferred when present.

Manual testing steps?

1. In a consuming app whose kiln bundle imports a crypto lib (crypto-browserify → safe-buffer), build JS: npx clay vite --only js.
2. Open a page in edit mode.
3. Before: Uncaught TypeError: Object prototype may only be an Object or null: undefined. After: edit mode boots normally.

Unit tests: npx jest lib/cmd/vite/plugins/browser-compat.test.js (green on Node 18 / 20 / 22 in CI).

Screenshots

N/A — console error on boot → clean boot.

Additional Context

Same failure family as the other Node-builtin stubs the Vite rollout has surfaced one at a time — each edit-mode code path that reaches a different builtin exposes the next incomplete stub. This change makes the buffer fallback robust for any Buffer.prototype subclasser, not just safe-buffer.

[coveralls]

coverage: 87.084%. remained the same — jordan/fix-buffer-stub into master