feat: Semantic DAG v0.6.0 — append-only causal graph with signing and sync

[ApiliumDevTeam]

Summary

Full implementation of the Semantic DAG (Directed Acyclic Graph) for AIngle, adding an append-only causal graph on top of the existing triple store — enabling full traceability, time-travel queries, and cross-node synchronization.

Phases included

1. Data model & persistent storage — DagAction, DagActionHash (BLAKE3), DagPayload (7 variants), DagTipSet, DagStore with pluggable backends (memory + Sled). Author and affected-triple indexes, schema versioning, index rebuild on restart. Pruning with 4 retention policies (KeepAll / KeepSince / KeepLast / KeepDepth).

2. Time-travel, export & sync protocol — State reconstruction at any point in the DAG via replay onto an ephemeral GraphDB. DagDiff for ancestry comparison. Portable export (DagGraph) with DOT (Graphviz), Mermaid, and JSON renderers. Pull-based sync protocol (SyncRequest / SyncResponse) with compute_missing and ingest.

3. GraphDB integration — 18 new public methods on GraphDB (feature-gated). Constructors: memory_with_dag(), sled_with_dag(). Methods: insert_via_dag(), delete_via_dag(), dag_tips(), dag_action(), dag_history(), dag_chain(), dag_prune(), dag_at(), dag_at_timestamp(), dag_sign(), dag_verify(), dag_export(), dag_ingest(), dag_compute_missing(), dag_diff(), etc. Full backward compatibility with existing insert() / delete().

4. WAL + Raft state machine — WalEntryKind::DagAction variant for serialization without circular dependencies. State machine applies actions to DagStore and materialized view. Snapshot includes dag_tips with backward compat (#[serde(default)]).

5. Write path through Raft — create_triple and delete_triple construct DagActions when the dag feature is enabled: determine tips → build action → serialize to WAL → Raft. AppState gains dag_author and dag_seq_counter. Startup creates a Genesis action if the DAG is empty.

6. REST endpoints — 12 new endpoints under /api/v1/dag/:

   • GET tips, action/:hash, history, chain, stats, at/:hash, diff, export
   • POST prune, sync, sync/pull
   • GET verify/:hash
   • Content-type negotiation for DOT / Mermaid / JSON on export

7. Mandatory Ed25519 signing — Every DagAction is signed with the node's Ed25519 key before Raft submission. The state machine rejects unsigned actions (Genesis exempt). Key derived from the existing P2P identity seed. Verification endpoint at GET /api/v1/dag/verify/:hash. Transparent to all exposed APIs.

Key changes

• 24 files changed, ~5,300 lines added
• New modules: aingle_graph::dag::* (action, backend, export, signing, store, sync, timetravel, tips, pruning)
• Feature gates: dag, dag-sign, sled-backend
• Crate dependencies: blake3, ed25519-dalek, rand_core, petgraph (dag feature only)

Test plan

• Verify compilation with cargo build --features dag
• Run unit tests: cargo test --features dag
• Confirm compilation without dag feature is unaffected
• Test REST endpoints against a test cluster
• Verify Ed25519 signing and rejection of unsigned actions
• Test time-travel and DOT/Mermaid export
• Validate pull-based sync between two nodes