Phase 0: build the world projection reducer for Forge sessions

[vultrdev]

Objective

Build the projector/reducer that consumes normalized runtime events and produces world state.

Scope

• event -> state projection
• deterministic updates
• task enters Forge
• agent activation changes
• artifact state changes
• approval state changes

References

• agentarium-docs/docs/architecture/frontend-state-model.md
• agentarium-docs/docs/design/forge-room.md
• agentarium-docs/docs/design/artifact-lifecycle.md
• agentarium-docs/docs/design/approval-flow.md

Done when

• a scripted session can move through the core state changes
• the reducer logic is testable and isolated from rendering