docs(rust): document compression begin boundary

Record that shared compression-begin dictionary selection now runs in Rust,
while C retains the reset, attach, and private dictionary operation callbacks.

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

rust/README.md

@@ -151,10 +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,
-CDict-begin parameter selection, initialization ordering, and source-window
-policy now run in Rust behind private-state bridges; their C callbacks and
-final begin operation remain. The remaining C paths must move before the
-rewrite is complete.
+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.
 Keeping that boundary explicit prevents a passing hybrid build from being
 mistaken for the final all-Rust result.