docs(rust): document advanced CDict boundary

Record that Rust now owns advanced CDict one-shot validation and begin/end
ordering while C retains the private operations behind callbacks.

Test Plan:
- Documentation-only change; code validation is covered by 8b783e8.

rust/README.md

@@ -152,7 +152,8 @@ public CCtx allocation validation and allocate-before-init ordering, public
 static-CCtx workspace validation and initialization dispatch, and public CDict
 constructor parameter selection, default-level normalization, and
 workspace/object teardown ordering, public static-CDict workspace validation
-and initialization dispatch, public
+and initialization dispatch, public advanced-CDict one-shot validation and
+begin/end ordering, public
 advanced-compression parameter validation/init policy, public
 usingDict parameter selection, dictionary-presence handling, and default-level
 normalization, and public usingCDict frame-policy construction and begin/end