docs(rust): document CDict initialization boundary

Clarify that CDict initialization ordering and scalar publication now run in
Rust, while allocation, private dictionary loading, and match/workspace
callbacks remain C-owned.

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

rust/README.md

@@ -147,13 +147,14 @@ 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 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.
+compression orchestration 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,
+CDict-begin parameter selection, initialization ordering, and source-window
+policy now run in Rust behind private-state bridges; their C callbacks and
+final begin operation remain. 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.