docs(rust): document CDict begin boundary

Record that CDict-begin parameter selection, initialization ordering, and
source-window policy now run in Rust while private CDict construction and the
final C begin operation remain on the C side of the migration boundary.

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

rust/README.md

@@ -146,10 +146,13 @@ compression-job stage sequencing and error flow, public sequence-API
 orchestration, sequence-store and block policy,
 external-sequence/literals block loop, optional-format decompression loops,
 block decoder wrappers, decompression result policy, and public end-of-frame
-compression orchestration now run in Rust. CDict lifecycle, the private
-dictionary-content loader, reset policy, private CCtx/matchfinder/workspace
-operations, and codec/adaptive-policy callbacks remain in C. The remaining C
-paths must move before the rewrite is complete.
+compression orchestration now run in Rust. CDict construction/lifecycle
+machinery, the private dictionary-content loader, reset policy, private
+CCtx/matchfinder/workspace operations, and codec/adaptive-policy callbacks
+remain in C. CDict-begin parameter selection, initialization ordering, and
+source-window policy now run in Rust behind a private-state bridge; its final
+C begin operation remains. 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.