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

lattice-core/src/entry.rs

@@ -0,0 +1,9 @@
+//! Log entries (atomic operations)
+
+/// An atomic, batched operation in the sigchain.
+///
+/// Entries are the fundamental unit of change in Lattice.
+/// The KV store is a "view" generated by replaying these entries.
+pub struct Entry {
+    // TODO: operation data, signature, prev_hash
+}