docs(rust): refresh migration boundary

Document the Rust-owned public end-of-frame compression orchestration and both multi-file compression scheduler boundaries, including the C callbacks that remain responsible for private file and codec operations.

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

rust/README.md

@@ -61,8 +61,9 @@ zstd ABI:
     Rust while C retains the private contexts and mutation callbacks.
     The public sequence APIs likewise use Rust-owned validation, frame-header,
     checksum, and output-accounting orchestration around C-owned block state.
-    The `ZSTD_compress2_c` fallback also uses a Rust-owned stable-buffer
-    orchestration boundary while C retains the context reset and stream adapter.
+    The public end-of-frame path and the `ZSTD_compress2_c` fallback also use
+    Rust-owned orchestration boundaries while C retains the context reset,
+    stream adapter, checksum/epilogue, and trace 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.
@@ -118,8 +119,10 @@ zstd ABI:
     asynchronous-pool adapters, metadata, zstd codec/error mapping, and
     adaptive-policy integration, while Rust owns the mixed-format probe/dispatch
     loop, zstd stream-compression I/O loop, scalar adaptive decisions,
-    optional-format decompression loops, and decompression result policy/final
-    accounting.
+    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.
     Rust already owns the file preference policy, filename decisions,
     source/destination opening, dictionary buffers, asynchronous I/O pools,
     and pass-through copy leaf.
@@ -134,16 +137,17 @@ zstd ABI:
 
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
 CLI zstd compression stream loop, scalar adaptive decisions, and the
-shared-destination multi-file compression scheduler,
+shared- and separate-destination multi-file compression schedulers,
 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,
 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
-CCtx/matchfinder/workspace operations, and codec/adaptive-policy callbacks
-remain in C. The remaining C paths must move before the rewrite is complete.
+block decoder wrappers, decompression result policy, and public end-of-frame
+compression orchestration now run in Rust. CDict lifecycle, the private
+dictionary-content loader, reset policy, private CCtx/matchfinder/workspace
+operations, and codec/adaptive-policy callbacks remain in C. 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.