docs(rust): document compress2 and multi-file boundaries

Record the Rust-owned stable-buffer fallback orchestration and shared-output
multi-file scheduler in the migration boundary documentation while retaining
the C-owned private context, resource, and callback responsibilities.

Test Plan:
- git diff --cached --check
- documentation-only boundary update; code validation is covered by
  17cfedd56 and 035001b76

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

rust/README.md

@@ -61,6 +61,8 @@ 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.
   - `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.
@@ -131,7 +133,8 @@ zstd ABI:
     tests do not compile.
 
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
-CLI zstd compression stream loop and scalar adaptive decisions,
+CLI zstd compression stream loop, scalar adaptive decisions, and the
+shared-destination multi-file compression scheduler,
 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