Contract declarations

#[sapio::contract] registers explicitly marked actions and spending policies. Ordinary helper methods remain ordinary helpers. #[policy] declares a policy that can guard actions; #[spend] also exports it as independently sufficient to spend the output.

Contract hooks are ordinary methods marked #[amount] for minimum funding, #[internal_key] for an already authorized Taproot internal key, and #[metadata] for descriptive object metadata. Selecting an internal key never grants new spending authority.

An explicit Contract implementation can register factories directly:

#![allow(unused)]
fn main() {
impl Contract for Escrow {
    declare! {actions, Self::refund, Self::propose_payment}
    declare! {finish, Self::cooperative}
}
}

Each action has its own request type before registration. A factory returns Option<Box<dyn ErasedAction<Self>>>; returning None explicitly omits an optional interface member. Action names must be unique within a contract so typed request paths cannot be silently redirected.

DynamicContract<S> assembles actions and independent finish factories in vectors alongside the contract data, metadata callback and minimum-funding callback. A custom AnyContract implementation can provide the same compiler interface without choosing a specific storage layout.

Existing addresses can be used through Compiled::from_address. They provide an output destination and minimum-funding information, but no action API or source policy beyond the supplied artifact.