feat: real serve path — framing, transport, Server (rollout step 7d)

[Kinflou]

Follows #3. Replaces the all-stubbed setup/ layer (async_trait + Arc<RwLock> + Box<dyn>, at odds with the sync / no-alloc contract) with a working one.

module
wire	request / response framing. no_std, no alloc — encode into a BufMut, borrow on decode. Request = [call_id u16][request_id u64][params]; Response = [request_id u64][envelope].
transport	a sync frame Transport trait (send / recv-into-buf) + an InMemory std impl over mpsc, with duplex().
serve	Server<D, W> — holds the Dispatch, the WireFormat, and three reused buffers; serve_one / serve read a request, dispatch, frame the response. Generic over the Transport.

tests/serve_roundtrip.rs — a Greet protocol served on a thread over InMemory while the client frames a request and reads the response, end to end.

setup/ and the old stubbed setups test are deleted, along with the setup-only deps (eyre, async-trait, downcast-rs, tokio, serde_json, json-rpc-types) and the json_rpc feature. package_abi stays.

Verified

All feature configs green: cargo test (12 lib + 3 + 1), --no-default-features (2), --features alloc (8). Warnings 7 → 1 (a pre-existing abi_stable macro lint in package_abi).

Next (7e)

The consumer side — a Client that owns a Transport + WireFormat, assigns request ids, and returns a borrowed Envelope; then a TCP Transport with length-prefixed stream framing.