docs(rust): refresh migration boundary

Update the Rust component map to describe the newly migrated single-threaded
stream state machine and multithreaded scheduler/flush policy. Correct the CLI
boundary so it no longer claims that the optional-format decompression loops
are Rust-owned, and list the remaining sequence, dictionary, and CLI leaves
that still block an all-Rust build.

Test Plan:
- `git diff --cached --check` -- passed.
- Documentation-only change; no code tests required.

rust/README.md

@@ -47,7 +47,10 @@ zstd ABI:
     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. The
+    private state-preparation and block-compression callbacks. Rust also owns
+    the single-threaded buffered/stable stream state machine, including direct
+    versus buffered output, pending-output draining, and frame reset policy.
+    The
     external-sequence-and-literals block loop is Rust-owned as well; C retains
     only its CCtx-dependent sequence-conversion callback.
   - `zstd_compress_frame` serializes frame headers, skippable frames, and the
@@ -79,7 +82,8 @@ zstd ABI:
   - `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, reusable input-range
-    overlap decisions, and pending-output decisions through scalar job
+    overlap decisions, outer scheduling and end-directive adjustments,
+    job-creation decisions, and pending-output decisions through scalar job
     projections.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
@@ -103,10 +107,10 @@ 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 format-specific codec callbacks and metadata,
+    `fileio` layer retains the format-specific codec callbacks, loops, and
-    while Rust owns the mixed-format probe/dispatch loop, decompression result
+    metadata, while Rust owns the mixed-format probe/dispatch loop and
-    policy/final accounting, and gzip, xz/lzma, and lz4 codec leaves. Rust
+    decompression result policy/final accounting. Rust already owns the file
-    already owns the file preference policy, filename decisions,
+    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`,
@@ -118,14 +122,15 @@ 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 and the
+Sequence-store construction and its no-compress fallback, remaining
-remaining multithreaded scheduling/outer-flush orchestration are still C. The
+CCtx-dependent block and sequence conversion, dictionary ingestion and CDict
-frame epilogue, produced-output flush policy, MT job-creation policy,
+lifecycle, optional-format decompression loops, and the CLI zstd compression
-reusable MT input-range policy, external-sequence/literals block loop, block
+leaf remain in C. The single-threaded buffered/stable stream state machine,
-decoder wrappers, decompression result policy, and gzip/xz/lzma/lz4 codec
+MT outer scheduling and flush policy, external-sequence/literals block loop,
-leaves now run in Rust. The remaining C paths must move before the rewrite is
+block decoder wrappers, and decompression result policy now run in Rust. The
-complete. Keeping that boundary explicit prevents a passing hybrid build from
+remaining C paths must move before the rewrite is complete. Keeping that
-being mistaken for the final all-Rust result.
+boundary explicit prevents a passing hybrid build from being mistaken for the
+final all-Rust result.
 
 ## Legacy decoding