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

lattice-core/src/sigchain.rs

@@ -0,0 +1,20 @@
+//! Cryptographic SigChain (append-only signed log)
+
+/// An append-only log where each entry is cryptographically signed
+/// and hash-linked to the previous entry.
+pub struct SigChain {
+    // TODO: entries, hash chain
+}
+
+impl SigChain {
+    /// Create a new empty sigchain.
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl Default for SigChain {
+    fn default() -> Self {
+        Self::new()
+    }
+}