docs(rust): document CDict free boundary

Record that Rust now owns CDict null handling and workspace/object teardown
ordering while C retains private workspace and allocator callbacks.

Test Plan:
- Documentation-only change; code validation is covered by 14157558f.

rust/README.md

@@ -149,7 +149,8 @@ external-sequence/literals block loop, optional-format decompression loops,
 block decoder wrappers, decompression result policy, and public end-of-frame
 compression orchestration, public CCtx-copy pledge/frame-parameter policy,
 public CCtx allocation validation and allocate-before-init ordering, and public
-CDict constructor parameter selection and default-level normalization, public
+CDict constructor parameter selection, default-level normalization, and
+workspace/object teardown 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
@@ -157,7 +158,7 @@ sequencing, legacy public CDict-begin frame-policy construction and
 unknown-source pledge, and the compressBegin_usingDict family’s unknown-source
 parameter selection and default-level normalization, and public advanced-begin
 parameter validation and init-then-begin ordering now run in Rust.
-CDict allocation/lifecycle
+CDict advanced 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,