feat: pluggable framing — Framing trait + JSON-RPC (7g)

[Kinflou]

The framing axis — how a call becomes bytes and back — is now a trait, orthogonal to WireFormat (which serialises the parts). Client<T, W, F> and Server<D, W, F> are generic over one, defaulting to the Comline datagram framing. The same dispatch / client code runs over JSON-RPC unchanged.

contract::Framing	encode/decode request + response; name() folds into the handshake
contract::DatagramFraming	the current binary framing, extracted — the top-level wire module is deleted, this is now the one impl of that byte layout
framing::JsonRpcFraming (std)	name-oriented: {"jsonrpc":"2.0","method":…,"params":…,"id":N}; a raised schema error → a JSON-RPC error object keyed by ordinal. Pairs with format::Json (new, serde_json)

Contract changes (need a companion codegen PR):

• Dispatch::dispatch — out: &mut dyn BufMut → reply: &mut Reply (a framing-agnostic ok / err / none sink) — the datagram Envelope layout is no longer baked into every dispatcher.
• Dispatch::calls() -> &'static [&'static str] — so a name-oriented framing's method can be resolved to an ordinal. Kind is unchanged; Server always hands the dispatcher Kind::Id.
• Client::call takes impl Into<Call> — a bare u16 for datagram-only callers, Call::new(id, name) from a generated stub.
• Client / Server: with_framing / connect_with_framing constructors.

Test

tests/jsonrpc_roundtrip.rs — the identical Greet stub over JsonRpcFraming + Json: a call, a typed raised error, and a byte assertion ({"jsonrpc":"2.0","method":"hello","params":{"name":"x"},"id":0}). Every existing roundtrip test updated to the Reply-based Dispatch. All feature configs green (--no-default-features, --features alloc, default); clippy clean.

Next

comline-codegen-rust: emit fn calls(), reply.ok()/reply.err() instead of Envelope::encode_*, Call::new(i, "name") in the client stub, and a framing type param / selector on the generated connect / serve helpers.