Framework adapters: request extraction and context construction beyond the axum example #32

Problem

The axum example is the only integration and hand-writes the whole handler (examples/axum/src/main.rs:342-383): deserializing greem::http::Request, constructing the application context, negotiating Accept, turning the payload stream into a response. #21 moves the response half into greem::http. The request half is still per application: how C is built per request, how connection state (headers, auth, peer address) reaches it, and GET requests, Content-Type dispatch and application/graphql-response+json are not handled at all.

State at 71ccf70

• Context<'req, C> is a view over the Arc<C> passed to execute_stream (greem/src/schema.rs:409-460); ticket What does a user write for one field, and what does codegen generate around it? #7 decided greem owns the handle and the application owns C. The example builds App once as router state.
• greem::http::Request has optional query, operationName, variables, extensions (greem/src/http.rs:9-19); no GET parsing, no batch shape.
• HTTP response helper: own the single-vs-multipart decision in greem::http #21's option C sketches adapter crates (greem-axum, ...) that could also own extraction; Support non-JSON wire formats (e.g. CBOR) #22 wants dispatch on the request's Content-Type.
• Schema::builder().stream_capacity(n) (schema.rs:128, default 100) is the only tuning knob; ticket How do @defer and @stream map onto generations and the arena? #12 left its default and any per-request override to "decide with real adapters".

Options

• Stay transport-agnostic: greem::http gains GET parsing and a plain RequestParts { method, content_type, accept, body } -> Request step; each integration writes the few lines around it.
• Adapter crates per framework (axum, actix-web, hyper) on top of the plain helpers, each with an extractor that builds C through a user closure over the request parts.
• Both: plain helpers in greem::http, one greem-axum crate as the reference adapter.

Open questions

• Is C built per request or per connection, and does greem need to know?
• Which GraphQL-over-HTTP rules does greem enforce (status codes for request errors, media types) versus the adapter?
• Should stream_capacity be overridable per request?

Blocked by #21.

Origin: Map: greem core, "Not yet specified" (request context construction, stream capacity tuning) and "Out of scope" (adapters beyond axum); tickets #7, #12, #14.