Compile-time DSL for the uniform thin JSON-RPC wrappers in Cartouche.RPC.

defrpc/3 generates, from one declaration, the four artifacts these wrappers otherwise hand-maintain in lockstep:

1. the public function,
2. its @spec (return type derived from :decode),
3. its @doc (carrying the doctest), and
4. the Descripex api/3 metadata block read by Cartouche.describe/2.

Two uniform shapes are covered, selected by whether :address_desc is given:

• address-at-block — name(<<_::160>> = address, opts) -> send_rpc(method, [encode(address), block_number], decode: ...) (e.g. get_balance, get_code).
• no-arg — name(opts) -> send_rpc(method, [], decode: ...) (e.g. eth_chain_id, gas_price).

Only the genuinely uniform shapes are covered. Wrappers with extra guards, struct dispatch, multi-clause normalization, or per-method prose deviations (e.g. get_nonce, whose param name, encoder, and opts description all differ) stay hand-written — folding them in would grow more knobs than the duplication it removes.

Prototype for ROADMAP Task 110 (defrpc), proving the api() introspection stays byte-identical across the conversion.