feat: adding kvbm-engine (#6773)

Signed-off-by: Ryan Olson <rolson@nvidia.com>

lib/velo-events/src/base/mod.rs → lib/kvbm-engine/src/leader/velo/mod.rs

 // SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 // SPDX-License-Identifier: Apache-2.0
 
-//! Core event storage engine backed by a generational slot system.
+mod service;
 
-pub(crate) mod system;
-
-pub use system::EventSystemBase;
+pub use service::{ExportMetadataCallback, VeloLeaderService};