feat: add initial

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/SQ-021-write-replication.md

@@ -0,0 +1,33 @@
+# SQ-021: Write Replication
+
+**Status:** `[ ] TODO`
+**Blocked by:** SQ-020, SQ-010
+**Priority:** High
+
+## Description
+
+Writes are replicated to N peers before ack to client. Simple quorum approach: coordinator writes locally, sends to peers, waits for majority ack.
+
+## Files to Create/Modify
+
+- `crates/sq-cluster/src/replication.rs` - Replicator with quorum logic
+- `crates/sq-server/src/grpc/cluster.rs` - ReplicateEntries RPC impl
+- `crates/sq-server/src/grpc/data_plane.rs` - update Publish to use Replicator
+
+## Replication Flow
+
+1. Coordinator receives Publish request
+2. Coordinator writes to local WAL, assigns offset
+3. Coordinator sends ReplicateEntries to all known alive peers
+4. Coordinator waits for W acks (W = floor(N/2) + 1, where N = replication factor)
+5. On quorum reached: ack to client
+6. On quorum timeout: return error to client
+
+## Acceptance Criteria
+
+- [ ] 3-node cluster: publish message, verify all 3 nodes have it in WAL
+- [ ] 3-node cluster, 1 node down: publish succeeds (2/3 quorum)
+- [ ] 3-node cluster, 2 nodes down: publish fails (no quorum)
+- [ ] ACK_MODE_LOCAL: ack after local WAL only (skip replication)
+- [ ] ACK_MODE_NONE: return immediately, replicate async
+- [ ] Replication timeout: configurable, default 5 seconds