docs(rust): document static CCtx boundary

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

Test Plan:
- Documentation-only change; code validation is covered by 463ceeb86.

rust/README.md

@@ -148,8 +148,9 @@ 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, 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
+public CCtx allocation validation and allocate-before-init ordering, public
+static-CCtx workspace validation and initialization dispatch, and public CDict
+constructor parameter selection, default-level normalization, and
 workspace/object teardown ordering, public static-CDict workspace validation
 and initialization dispatch, public
 advanced-compression parameter validation/init policy, public
@@ -159,8 +160,9 @@ 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 static-CDict workspace
-construction and dictionary-content allocation/loading, reset policy,
+CDict advanced allocation/lifecycle machinery, private static-CCtx and
+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