Implement Linux storage, state restoration, and action execution

[sij411]

Description

Add durable Linux runtime state and execute the non-network actions returned by FederCore.

Scope

• Define a simple Linux storage backend suitable for the preview runtime.
• Persist follower relations, delivery targets, local objects, and required activity metadata.
• Execute StoreFollower, StoreDeliveryTarget, and StoreObject actions idempotently.
• Restore persisted followers, delivery targets, objects, and activities when constructing FederCore.
• Record enough pending delivery-job state for later outgoing execution.
• Define clear transaction/error behavior when only part of an action list succeeds.

Done when

• Runtime state survives process restart.
• Restored core state produces the same relevant decisions as before restart.
• Repeated storage actions do not create duplicate relations or targets.
• Tests cover persistence, restoration, target refresh, and partial-failure behavior.

Out of scope

• HTTP Signature operations.
• Remote actor resolution.
• Sending activities over the network.
• Production-scale database design or migrations.