Core Input/Action boundary

[sij411]

Description

Define the core/runtime boundary so the core can make decisions without doing I/O.

Proposal

• Add Input, Action, HandleResult
• Implement FederCore::handle(input) -> HandleResult
• Inject nondeterminism via inputs/config (ids/timestamps)

Done when

• Core returns action lists for inputs
• Core performs no real I/O

Out of scope

• tokio/reqwest/hyper/axum
• filesystem/db access
• SystemTime::now() inside core