Route Directed Evolution worker results through WorkerRuns

[nerdsane]

Summary

• move Directed Evolution local worker execution from legacy BrainRuns to PawOrchestration WorkerRuns
• route completed WorkItems through Directed Evolution WorkItemReceipts so observer results materialize as domain entities
• link EvidenceArtifacts to WorkerRun ids and cover the contract with unit tests

Verification

• cargo test -p paw-codex-worker --locked
• cargo check -p paw-codex-worker --locked
• git diff --check
• Railway deployment 4d62ff70-45b3-44f4-a42b-0d29d7be1b8a reached SUCCESS

[rita-aga]

Superseded by #393, which folds these two commits in and fixes the success-path receipt-routing ordering (a routing failure could strand the WorkItem in Running with a succeeded WorkerRun) plus adds Running-WorkItem boot recovery. All Directed Evolution worker work for this effort continues in #393.