docs(rust): refresh migration boundary

Document the newly Rust-owned external-sequence block loop, MT input-range
policy, block-decoder wrappers, and CLI decompression result policy so the
remaining C orchestration boundary stays explicit.

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

rust/README.md

@@ -47,7 +47,9 @@ 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.
+    private state-preparation and block-compression callbacks. 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
     last empty block; it takes scalar frame parameters so the C-owned
     `ZSTD_CCtx_params` layout never crosses the language boundary.
@@ -76,8 +78,9 @@ zstd ABI:
   - `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.
+    private to C while Rust owns input-retention scans, reusable input-range
+    overlap decisions, and pending-output decisions through scalar job
+    projections.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 - Legacy decoding
@@ -89,7 +92,8 @@ zstd ABI:
   - `zstd_decompress_block` decodes literal and sequence sections, maintains
     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.
+    state. C retains the decoder-context layout and configuration projection;
+    the public and fullbench block-decoder wrappers are Rust-owned.
   - `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
@@ -100,10 +104,11 @@ zstd ABI:
     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,
-    while Rust owns the mixed-format probe/dispatch loop and gzip, xz/lzma,
-    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.
+    while Rust owns the mixed-format probe/dispatch loop, decompression result
+    policy/final accounting, and gzip, xz/lzma, 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`,
     while `benchfn` owns the benchmark run/timing loop (`BMK_benchFunction`,
     `BMK_benchTimedFn`) and `benchzstd` owns benchmark orchestration and
@@ -115,10 +120,12 @@ zstd ABI:
 
 Generic high-level frame-compression state-machine orchestration and the
 remaining multithreaded scheduling/outer-flush orchestration are still C. The
-frame epilogue, produced-output flush policy, MT job-creation policy, and
-gzip/xz/lzma/lz4 codec leaves now run in Rust. The remaining C paths must move
-before the rewrite is complete. Keeping that boundary explicit prevents a
-passing hybrid build from being mistaken for the final all-Rust result.
+frame epilogue, produced-output flush policy, MT job-creation policy,
+reusable MT input-range policy, external-sequence/literals block loop, block
+decoder wrappers, decompression result policy, and gzip/xz/lzma/lz4 codec
+leaves now run in Rust. The remaining C paths must move before the rewrite is
+complete. Keeping that boundary explicit prevents a passing hybrid build from
+being mistaken for the final all-Rust result.
 
 ## Legacy decoding