docs(rust): refresh migration boundary

Document that the Rust CLI now owns the gzip, xz/lzma, and LZ4 codec leaves as
well as MT job creation, while the generic frame-compression state machine and
remaining MT scheduling/outer-flush orchestration remain C-owned. Keep the
hybrid boundary explicit so compatibility-suite success is not mistaken for
completion of the full rewrite.

Test Plan:
- git diff --cached --check
- reviewed rust/README.md against the committed Rust/C ownership boundary

rust/README.md

@@ -99,9 +99,9 @@ zstd ABI:
   - `zstd_cli` owns the Rust parser, safety policy, and dispatch.  It is built
     by the separate `cli/` static-library package only for program archives,
     so library builds do not acquire program-only dependencies.  The C
-    `fileio` layer retains the remaining format-specific codec callbacks and
+    `fileio` layer retains the format-specific codec callbacks and metadata,
-    metadata, while Rust owns the mixed-format probe/dispatch loop and gzip
+    while Rust owns the mixed-format probe/dispatch loop and gzip, xz/lzma,
-    codec leaf. Rust already owns the file
+    and lz4 codec leaves. Rust already owns the file
     preference policy, filename decisions, source/destination opening,
     dictionary buffers, asynchronous I/O pools, and pass-through copy leaf.
   - `timefn` provides the monotonic nanosecond clock behind `UTIL_time_t`,
@@ -113,12 +113,12 @@ zstd ABI:
     feature, because the parser layer requires the C `fileio` backend that
     tests do not compile.
 
-Generic high-level frame-compression state-machine orchestration,
+Generic high-level frame-compression state-machine orchestration and the
-multithreaded job scheduling and the remaining outer flush orchestration, and
+remaining multithreaded scheduling/outer-flush orchestration are still C. The
-the xz/lzma/lz4 CLI codec paths are still C. The frame epilogue, produced-output
+frame epilogue, produced-output flush policy, MT job-creation policy, and
-flush policy, and gzip codec leaf now run in Rust. The remaining C paths must
+gzip/xz/lzma/lz4 codec leaves now run in Rust. The remaining C paths must move
-move before the rewrite is complete. Keeping that boundary explicit prevents
+before the rewrite is complete. Keeping that boundary explicit prevents a
-a passing hybrid build from being mistaken for the final all-Rust result.
+passing hybrid build from being mistaken for the final all-Rust result.
 
 ## Legacy decoding