feat: Introduce Hashline encoding/validation, implement core REPL and chat session logic, and establish initial project structure with a license and editor configuration.

Hashline/HashlineEncoder.cs

@@ -71,6 +71,8 @@ internal static class HashlineEncoder
     /// Computes a short file-level fingerprint: XOR of all per-line hashes (as bytes).
     /// Useful for cheap full-file staleness checks.
     /// </summary>
+    /// <param name="lines">All lines of the file (without trailing newlines).</param>
+    /// <returns>A 2-character hex fingerprint.</returns>
     public static string FileFingerprint(string[] lines)
     {
         int fp = 0;