docs(rust): document static CDict boundary

Record that Rust now owns static CDict workspace validation and dispatch while
C retains private workspace construction and dictionary initialization.

Test Plan:
- Documentation-only change; code validation is covered by 12b99469b.

rust/README.md

@@ -150,7 +150,8 @@ 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, default-level normalization, and
-workspace/object teardown ordering, public
+workspace/object teardown ordering, public static-CDict workspace validation
+and initialization dispatch, 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
@@ -158,8 +159,8 @@ 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 advanced allocation/lifecycle
-machinery, private dictionary-content allocation/loading, reset policy,
+CDict advanced allocation/lifecycle machinery, private static-CDict workspace
+construction and 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, CDict reset attach-versus-copy