refactor: simplify cf workers

[alukach]

What I'm changing

Writing a Cloudflare Workers consumer of multistore required too much boilerplate: every handler manually parsed web_sys::Request into method/path/query/headers, every forward() impl downcast Box<dyn Any> to recover its concrete body type, and every backend rebuilt web_sys::Headers from a HeaderMap by hand. Two trait bounds also forced WASM workarounds: ProxyBackend::forward was generic over Body, and BucketRegistry / CredentialRegistry required Send + Sync — which meant any registry holding a !Send type (e.g. worker::Fetch, CF Cache API) needed spawn_local + oneshot channel bridges to fake Send.

This PR removes that boilerplate, tightens the cf-workers public surface, and rebases the trait bounds so WASM runtimes can hold !Send types directly. The end result: the cf-workers example handler shrinks to a three-line call chain.

How I did it

Core trait changes (consumed by every runtime):

• crates/core/src/backend/mod.rs — ProxyBackend::forward drops the <Body: MaybeSend> generic in favour of an associated type Body. Body-type mismatches now fail at compile time instead of via Box<dyn Any> downcast at runtime. Touches every implementer: WorkerBackend, ServerBackend, LambdaBackend.
• crates/core/src/registry/{bucket,credential}.rs — BucketRegistry and CredentialRegistry change from Send + Sync to MaybeSend + MaybeSync, matching the existing ProxyBackend / Middleware / RouteHandler bounds. WASM registries can now hold worker::Fetch and the CF Cache API directly.
• crates/core/src/route_handler.rs — new HandlerAction::response_headers_mut() returns Option<&mut HeaderMap> for the Response / Forward variants. Middleware that injects CORS or Server-Timing headers no longer needs a three-arm match.
• crates/core/src/proxy.rs — ProxyGateway::handle_request updated to use B::Body instead of a generic, propagating the trait change.

cf-workers crate ergonomics:

• crates/cf-workers/src/request.rs — new RequestParts struct: from_web_sys() extracts owned method, path, query, headers, source_ip plus the body JsBody; as_request_info() borrows them as a RequestInfo<'_> for handle_request. Replaces ~8 lines of per-handler parsing.
• crates/cf-workers/src/response.rs — new GatewayResponseExt trait adds .into_web_sys() directly on GatewayResponse, replacing the free response_from_gateway function so IDE autocomplete surfaces it on the result type.
• crates/cf-workers/src/headers.rs — new WsHeaders newtype with From<&HeaderMap>. The newtype exists to dodge the orphan rule (From on foreign web_sys::Headers). Used by WorkerBackend::forward and WorkerBackend::send_raw, deleting four hand-rolled header-conversion blocks and the now-dead headers_to_js.
• crates/cf-workers/src/body.rs — JsBody's inner Option<ReadableStream> is now private behind new() / stream() accessors, preventing accidental mutation of the borrowed stream.
• crates/cf-workers/src/cors.rs — new add_cors_headers(&mut HeaderMap) helper for the common read-only-public-proxy case. Pairs naturally with HandlerAction::response_headers_mut() above.
• crates/cf-workers/src/request.rs — from_web_sys now percent-decodes the URL path via percent-encoding, since object keys with spaces / Unicode arrive URL-encoded from browsers.
• crates/cf-workers/src/lib.rs — trim public exports from 8 to 2 (headermap_from_js, GatewayResponseExt). Removed dead xml_response / s3_error_response; demoted internal helpers to pub(crate).

Other:

• docs/architecture/crate-layout.md — describe the new cf-workers exports (RequestParts, GatewayResponseExt, JsBody, WsHeaders) and the GatewayResponseExt::into_web_sys() flow.
• examples/cf-workers/src/lib.rs — collapse the handler body to gateway.handle_request(...).await.into_web_sys() to demonstrate the new API surface end-to-end.
• Last commit (chore: merge fix) repairs the merge from main: the ForwardMockBackend test that came in with PR feat(core): add User-Agent header to outbound backend requests #20 still used the old forward<Body: MaybeSend> signature, so it gained type Body = (); and dropped the generic.

Test plan

• make ci (fmt + clippy -D warnings + cargo check + cargo check -p multistore-cf-workers --target wasm32-unknown-unknown + full test suite)

[github-actions]

🚀 Latest commit deployed to https://multistore-proxy-pr-47.development-seed.workers.dev

• Date: 2026-05-21T05:27:19Z
• Commit: 021882e