Minimal FederCore state

[sij411]

Description

Add the minimal in-memory state needed for Phase 1 flows.

Proposal

• Implement FederState (or equivalent) owned by FederCore
• Track local actor config
• Track followers + delivery targets
• Track created objects/activities (as needed for tests)

Done when

• FederCore::new(config) works
• Tests can create core+state and mutate via inputs

Out of scope

• Persistence/storage backend
• Config file parsing