feat: Bundle size improvements for prefetch config

[Friss]

• Replace package.json require with SDK_VERSION build-time constant
• Add separate prefetch entry point for smaller bundle imports
• Enable code splitting in tsup for tree-shaking
• Add ./prefetch subpath export in package.json
• Export as es2020
• Remove uuid dep
• Full bundle: ~14.8KB, Prefetch-only: ~3.2KB (78% smaller)

[jdwyah]

i think this isn't a uuid.

maybe

function uuid() {
  if (typeof crypto !== "undefined" && crypto.randomUUID) {
    return crypto.randomUUID();
  }

  // Fallback: not cryptographically secure, but at least a valid UUIDv4 format
  return "xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx".replace(/[xy]/g, (c) => {
    const r = (Math.random() * 16) | 0;
    const v = c === "x" ? r : (r & 0x3) | 0x8;
    return v.toString(16);
  });
}