docs(rust): document shared decompression scheduler

Record the shared-destination decompression scheduler as a Rust-owned boundary while documenting that C retains destination, resource, and format callbacks.

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

rust/README.md

@@ -121,8 +121,9 @@ zstd ABI:
     loop, zstd stream-compression I/O loop, scalar adaptive decisions,
     optional-format decompression loops, decompression result policy/final
     accounting, and both shared- and separate-destination multi-file
-    compression schedulers. C retains destination-name construction and the
-    private file/resource callbacks for the separate-destination path.
+    compression schedulers, plus the shared-destination decompression
+    scheduler. C retains destination-name construction and the private
+    file/resource/format callbacks for these scheduler boundaries.
     Rust already owns the file preference policy, filename decisions,
     source/destination opening, dictionary buffers, asynchronous I/O pools,
     and pass-through copy leaf.
@@ -138,6 +139,7 @@ zstd ABI:
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
 CLI zstd compression stream loop, scalar adaptive decisions, and the
 shared- and separate-destination multi-file compression schedulers,
+shared-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