docs(rust): document CDict reset boundary

Record that Rust now selects the CDict attach-versus-copy reset path while C
retains the workspace- and match-state-heavy reset operations.

Test Plan:
- git diff --cached --check

rust/README.md

@@ -151,11 +151,11 @@ compression orchestration now run in Rust. CDict allocation/lifecycle
 machinery, private dictionary-content allocation/loading, reset policy,
 private CCtx/matchfinder/workspace operations, and codec/adaptive-policy
 callbacks remain in C. CDict initialization ordering and scalar publication,
-shared compression-begin dictionary selection, and CDict-begin parameter
-selection, initialization ordering, and source-window policy now run in Rust
-behind private-state bridges; their C callbacks and final begin/reset/attach
-operations remain. The remaining C paths must move before the rewrite is
-complete.
+shared compression-begin dictionary selection, CDict reset attach-versus-copy
+selection, and CDict-begin parameter selection, initialization ordering, and
+source-window policy now run in Rust behind private-state bridges; their C
+callbacks and final begin/reset/attach/copy operations remain. The remaining
+C paths must move before the rewrite is complete.
 Keeping that boundary explicit prevents a passing hybrid build from being
 mistaken for the final all-Rust result.