Merge pull request #201 from decipherhub/docs/adr-013-commonware-aggregation

docs: add ADR-013 for Commonware Aggregation

Author: qj0r9j0vc2

docs/architecture/adr-013-commonware-aggregation.md

@@ -0,0 +1,75 @@
+# ADR 013: Commonware Aggregation for Fast Consensus Sync
+
+## Changelog
+
+* 2026-02-01: Initial draft
+
+## Status
+
+PROPOSED Not Implemented
+
+## Abstract
+
+Adopt Commonware Aggregation to generate State Root Certificates for O(1) consensus sync. Reduces sync time from ~20 minutes to ~2.5 minutes for 2,300 blocks.
+
+## Context
+
+Current consensus sync validates blocks sequentially (~500ms × 2,300 blocks = ~20 min). The bottleneck is per-block cryptographic verification. Snap sync downloads state efficiently but lacks a trust anchor without full block validation.
+
+## Alternatives
+
+| Alternative | Pros | Cons |
+|-------------|------|------|
+| Parallel validation | No dependencies | O(n), limited by sequential state |
+| Hardcoded checkpoints | Simple | Centralized, stale between releases |
+| Light client proofs | Trustless | Complex, high bandwidth |
+| **Aggregation** | **O(1), consensus-agnostic** | **New dependency** |
+
+## Decision
+
+Adopt Commonware Aggregation to create quorum certificates over state roots.
+
+### Core Mechanism
+
+```rust
+// Validators sign state roots after each block
+struct Item { height: u64, payload: B256 }  // state root
+struct Certificate { item: Item, signers: BitVec, signature: Signature }
+```
+
+### Sync Flow
+
+| Current | With Aggregation |
+|---------|------------------|
+| Validate 2,300 blocks sequentially | Verify single certificate |
+| ~20 minutes | ~2.5 minutes |
+
+### Implementation
+
+1. **Broadcast**: After block finalization, broadcast state root
+2. **Acknowledge**: Validators sign and return acks
+3. **Aggregate**: Engine produces certificate at 2/3+ quorum
+4. **Sync**: New nodes verify certificate, then snap sync state
+
+### Consensus Agnosticism
+
+Aggregation only requires validator set + BLS keys. Works with Malachite now, Simplex later.
+
+## Consequences
+
+### Positive
+
+- 10x sync speedup
+- O(1) verification
+- Consensus-agnostic
+- Reuses existing snap sync and BLS
+
+### Negative
+
+- Protocol complexity (new message types)
+- Requires 2/3+ online validators
+- New Commonware dependency
+
+## References
+
+* [Commonware Aggregation](https://github.com/commonwarexyz/monorepo/tree/main/consensus/src/aggregation)