feat: consumer-side Client + a TCP transport (rollout step 7e)

[Kinflou]

Follows #4 — the mirror of 7d's Server, and a real stream transport under it.

module
client	Client<T, W> — owns a Transport + a WireFormat, hands out request ids, reuses its send / recv buffers. call(call_id, &params) frames the header, serializes params straight in after it (no scratch buffer), sends, blocks for the response, and returns (Envelope<'_>, &W) — both views out of one &mut self borrow, so the generated stub decodes Ok as R or maps an Err ordinal without reaching back into the client. One outstanding call at a time; pipelining is a later, additive layer.
transport	Tcp — a Transport over a TCP byte stream. A stream has no message boundaries, so every frame gets a u32 LE length prefix; recv reads exactly one and rejects a prefix over MAX_FRAME (16 MiB) before allocating. Tcp::new wraps an accepted stream, Tcp::connect dials.
wire	encode_request_header split out of encode_request so a client can frame the header then encode params into the same buffer.

tests/client_roundtrip.rs — a Greet protocol (with a raised schema error) driven through a Client-backed stub against a Server, once over InMemory and once over loopback Tcp, plus a check that back-to-back Tcp frames keep their boundaries.

Client is alloc-gated (Vec buffers) like serve / transport; Tcp is std-gated like InMemory.

Verified

All feature configs green: cargo test (13 lib + 3 client + 3 dispatch + 1 serve), --no-default-features (2), --features alloc (9). Warnings unchanged (1 — the pre-existing abi_stable macro lint).

Next (7f)

Fold JSON-RPC back in as a framing option (name-oriented Kind, {jsonrpc, method, params, id} envelope) selectable under the same Client / Server; then the async (std) layer.