Document cycle safeguards (#25)

Clarify that cycle creation via add is prevented, removing the old to_s/size
infinite-loop risk, and point readers to validate_acyclic!/acyclic? for
untrusted input checks.

Author: evolve75

History.md

@@ -7,6 +7,12 @@
 * Treat `nil` child slots as empty in sibling/child checks to avoid phantom
   nodes in sparse binary trees.
 
+* Add explicit cycle validation helpers (`validate_acyclic!`, `acyclic?`) for
+  untrusted input.
+
+* Cycle prevention in `add` eliminates the earlier `to_s`/`size` infinite-loop
+  risk caused by cyclic graphs.
+
 ### 2.2.1pre / 2026-02-07
 
 * Simplified development dependency constraints while maintaining Ruby 2.7+

README.md

@@ -44,6 +44,9 @@ See [History](./History.md) for the detailed Changelog.
 See [API-CHANGES](./API-CHANGES.md) for the detailed description of
 API level changes.
 
+Cycle creation via `add` is prevented, and explicit cycle validation helpers
+are available for untrusted input (see `validate_acyclic!` and `acyclic?`).
+
 ## GETTING STARTED: ##
 
 This is a basic usage example of the library to create and manipulate a tree.