docs(rust): document CCtx copy boundary

Record that Rust now owns public CCtx-copy pledge and frame-parameter policy,
while C retains the private workspace and table-copy operation.

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

rust/README.md

@@ -147,7 +147,8 @@ orchestration, sequence-store and block policy, external-producer invocation
 and success-path validation, external-sequence-store reset,
 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 allocation/lifecycle
+compression orchestration, and public CCtx-copy pledge/frame-parameter policy
+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,