feat(cloudflare): add Cloudflare runtime srcset builder

[dangreen]

Adds @srcset/cloudflare - runtime srcset builder for Cloudflare image transformations urls, mirroring @srcset/imgproxy (#13).

• Cloudflare - isomorphic builder of loader-shaped srcset objects (url, src, srcSet, srcMap) for content images, e.g. from an API or a CMS; zero configuration on a Cloudflare zone (/cdn-cgi/image endpoint by default), custom endpoint for another zone
• Only forceable output formats (jpeg, webp, avif) are accepted; a non-forceable format is valid only as the source format passing through, otherwise the builder throws
• An svg source always passes through: Cloudflare returns svg as is, ignoring transformations
• Custom transformations via the processing hook, e.g. format=auto negotiation by the Accept header
• Passthrough mode for local development outside of a Cloudflare zone
• No url signing (Cloudflare has none) - no server-only entry
• Reviewed locally with Codex: svg semantics, non-forceable formats and docs findings addressed

🤖 Generated with Claude Code