feat: Introduce Hybrid Logical Clocks in protobuf, update architecture documentation, and add HLC test cases.

lattice-store/src/watermark.rs

@@ -0,0 +1,9 @@
+//! Watermarks for agreeing on safe compaction points
+
+/// A watermark representing a safe cut-off point for log compaction.
+///
+/// Nodes use watermarks to agree on which entries can be safely
+/// deleted and replaced with snapshots.
+pub struct Watermark {
+    // TODO: watermark data
+}