docs(rust): refresh migration map

Record the new ownership boundaries for frame-chunk compression,
multithreaded job decisions, sequence decoder selection, and mixed-format
CLI dispatch. Keep the remaining C-owned orchestration explicitly visible
while the rewrite continues.

Test Plan:
- git diff --cached --check
- cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check
- cargo +nightly fmt --manifest-path rust/cli/Cargo.toml -- --check

rust/README.md

@@ -44,7 +44,10 @@ zstd ABI:
     and the outer block-dispatch decision. `zstd_compress` also owns ordinary
     sequence-block entropy emission, sequence collection, and the legacy RLE
     compatibility gate after C builds the sequence store; C retains the
-    sequence-store builder and no-compress fallback.
+    sequence-store builder and no-compress fallback. Rust also owns the
+    frame-chunk block loop, including block sizing, target/split/internal
+    dispatch, output accounting, and frame-state updates; C supplies the
+    private state-preparation and block-compression callbacks.
   - `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.
@@ -71,6 +74,9 @@ zstd ABI:
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.
+  - The `zstdmt_compress` integration keeps job descriptors and synchronization
+    private to C while Rust owns input-retention scans and pending-output
+    decisions through scalar job projections.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 - Legacy decoding
@@ -80,7 +86,9 @@ zstd ABI:
     shims for the native build.
 - Block decompression
   - `zstd_decompress_block` decodes literal and sequence sections, maintains
-    FSE/Huffman repeat state, and executes compressed-block sequences.
+    FSE/Huffman repeat state, executes compressed-block sequences, and selects
+    the short or long sequence decoder from projected configuration and history
+    state. C retains the decoder-context layout and configuration projection.
   - `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
@@ -90,8 +98,8 @@ 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 still owns format-specific dispatch, metadata, and the
-    remaining C streaming/orchestration paths.  Rust already owns the file
+    `fileio` layer retains format-specific codec callbacks and metadata, while
+    Rust owns the mixed-format probe/dispatch loop. 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`,