docs(rust): document external sequence boundaries

Record that Rust now owns external-producer invocation and successful-path
validation plus external raw-sequence-store reset, while C retains private
transfer, fallback, and context operations.

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

rust/README.md

@@ -143,7 +143,8 @@ shared- and separate-destination decompression scheduling,
 single-threaded stream initialization and the buffered/stable stream state
 machine, MT stream initialization, MT outer scheduling and flush policy, MT
 compression-job stage sequencing and error flow, public sequence-API
-orchestration, sequence-store and block policy,
+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