docs(rust): document stream initialization boundary

Document that Rust owns single-threaded and MT stream-initialization policy,
including the ordered setup decisions and scalar projections, while C retains
private contexts and mutation callbacks.

Test Plan:
- git diff --cached --check
- documentation-only boundary update; code validation is covered by the
  preceding stream-initialization commits

Commit is intentionally unsigned because GPG pinentry hangs in this
non-interactive environment.

rust/README.md

@@ -55,6 +55,10 @@ zstd ABI:
   - `zstd_compress_frame` serializes frame headers, skippable frames, and the
     last empty block; it takes scalar frame parameters so the C-owned
     `ZSTD_CCtx_params` layout never crosses the language boundary.
+  - `zstd_compress` also owns the transparent single-threaded and multithreaded
+    stream-initialization policy: dictionary selection, parameter resolution,
+    initial buffer sizing, and the ordered setup decisions are projected into
+    Rust while C retains the private contexts and mutation callbacks.
   - `zstd_compress_params` owns the compression-level tables (formerly
     `clevels.h`), parameter bounds, clamping, validation, table selection,
     source/dictionary adjustment, and match-state/CDict size estimation.
@@ -124,9 +128,10 @@ zstd ABI:
     tests do not compile.
 
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
-CLI zstd compression stream loop, the single-threaded buffered/stable stream
-state machine, MT outer scheduling and flush policy, MT compression-job stage
-sequencing and error flow, sequence-store and block policy,
+CLI zstd compression stream loop, 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, sequence-store and block policy,
 external-sequence/literals block loop, optional-format decompression loops,
 block decoder wrappers, and decompression result policy now run in Rust. CDict
 lifecycle, the private dictionary-content loader, reset policy, private