[ld1287]

What

Closes #56 by adding a runtime layer on top of the closed 32-entry
catalog::ANSWERS table. Hosts (e.g. finance-brief with 17 capabilities)
can now call catalog::register(SysContract { ... }) at boot to expose
their own sys.* helpers, without forking octoscript-ui-l0.

API

pub fn register(contract: SysContract) -> Result<&'static SysContract, String>;
pub fn registered_names() -> Vec<String>;