fix(cluster): sign DAG actions on cluster nodes + deterministic genesis (#99)

[ApiliumDevTeam]

Summary

Fixes #99 — cluster DAG writes were rejected with 500 — "DagAction rejected: missing Ed25519 signature", failing the cluster integration tests.

Two root causes, both fixed:

1. Signing setup was binary-only. The DAG enable + author + Ed25519 signing-key initialization lived only in main(). Library/test callers that go through cluster_init::init_cluster (bypassing main) got an un-enabled, unsigned DAG, so cluster writes were emitted unsigned and rejected by the Raft state machine. → Extracted into cluster_init::ensure_dag_ready, called from init_cluster and from main (standalone mode only, to avoid double init).

2. Genesis hash diverged across nodes (exposed once deps(rust): bump tokio-tungstenite from 0.20.1 to 0.28.0 #1 was fixed). Each node built its DAG genesis with a wall-clock Utc::now() timestamp, so genesis hashes differed per node; a follower then failed to validate a replicated action's parent (the leader's genesis), since DagStore::put requires every parent to exist locally — so the triple never materialized on followers. → Made the genesis deterministic (fixed epoch timestamp) so every fresh node computes the same genesis hash and cross-node replication validates. Existing persistent DAGs are unaffected (init_or_migrate is a no-op once actions exist).

Also hardened test_three_node_cluster_replication to poll for replication (up to 15s) instead of assuming a fixed 2s delay.

Test plan

• cargo test -p aingle_cortex --features dag --test cluster_integration_test — 3/3 pass (single_node_bootstrap, three_node_replication, wal_stats); previously 1/3.
• cargo test -p aingle_graph --features dag dag — 76 pass (deterministic genesis breaks no existing DAG test).
• cargo build -p aingle_cortex (default) and --features dag — compile cleanly.

Notes

• Discovered during the final verification of feat(cortex): MCP stdio server (--mcp) exposing the semantic graph to LLMs #98 (MCP server); independent of that work.
• Touches aingle_graph genesis semantics (core crate) — the change only affects newly-created genesis actions and is required for cluster DAG consistency.