docs(rust): record migrated policy seams

Keep the migration boundary accurate after moving frame-progression dispatch, the decompression no-forward-progress threshold, and compression metadata-transfer selection into Rust. The README now states which scalar inputs Rust owns and which private callbacks, probes, and operations remain in C.

Test Plan:

- git diff --cached --check

- Capped clippy, CLI tests, native build, smoke test, and full upstream suite passed before this documentation-only commit

rust/README.md

@@ -68,6 +68,9 @@ zstd ABI:
     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.
+    Public frame-progression dispatch is Rust-owned as well; C projects the
+    scalar single-thread inputs and keeps the private MT context behind a
+    callback.
   - `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.
@@ -119,9 +122,9 @@ zstd ABI:
   - `zstd_decompress` owns the public decompression context, one-shot,
     dictionary, parameter, and streaming state machines.  Its C shim retains
     the configuration-dependent context layout and platform details, plus
-    legacy and trace leaves; Rust owns decoder storage allocation, custom
-    memory dispatch, and the `ZSTD_copyDCtx` prefix copy up to the projected
-    `inBuff` field.
+    legacy and trace leaves; Rust owns the no-forward-progress threshold
+    policy, decoder storage allocation, custom memory dispatch, and the
+    `ZSTD_copyDCtx` prefix copy up to the projected `inBuff` field.
 - Command-line frontend
   - `zstd_cli` owns the Rust parser, safety policy, and dispatch.  It is built
     by the separate `cli/` static-library package only for program archives,
@@ -133,8 +136,10 @@ zstd ABI:
     optional-format decompression loops, decompression result policy/final
     accounting, both shared- and separate-destination multi-file compression
     schedulers, and both shared- and separate-destination decompression
-    schedulers. C retains destination-name construction and the private
-    file/resource/format callbacks for these scheduler boundaries.
+    schedulers. Rust also owns the scalar compression metadata-transfer
+    decision; C retains the private stat probe, metadata operations,
+    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.