docs(rust): refresh migration boundary

Document that Rust now owns sequence-store construction, block post-build
policy, public sequence conversion, and optional-format decompression loops.
Keep the remaining private C callback and dictionary/CLI boundaries explicit so
the hybrid state is not mistaken for a completed all-Rust rewrite.

Test Plan:
- git diff --check (passed)

rust/README.md

@@ -42,17 +42,16 @@ zstd ABI:
     partitions, while `zstd_compress` emits those partitions through the
     Rust single-block serializer. C retains split discovery's context setup
     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. 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. 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.
+    sequence-block entropy emission, sequence collection, the legacy RLE
+    compatibility gate, and sequence-store construction; C supplies only the
+    private matchfinder, LDM, external-sequence-producer, and state-preparation
+    callbacks. 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 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 and public sequence
+    conversion are Rust-owned as well; C retains only CCtx-facing adapters.
   - `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.
@@ -107,10 +106,11 @@ 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, loops, and
-    metadata, while Rust owns the mixed-format probe/dispatch loop and
-    decompression result policy/final accounting. Rust already owns the file
-    preference policy, filename decisions,
+    `fileio` layer retains the format-specific codec callbacks, private
+    asynchronous-pool adapters, and metadata, while Rust owns the mixed-format
+    probe/dispatch loop, optional-format decompression loops, and decompression
+    result policy/final accounting. 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`,
@@ -122,15 +122,14 @@ zstd ABI:
     feature, because the parser layer requires the C `fileio` backend that
     tests do not compile.
 
-Sequence-store construction and its no-compress fallback, remaining
-CCtx-dependent block and sequence conversion, dictionary ingestion and CDict
-lifecycle, optional-format decompression loops, and the CLI zstd compression
-leaf remain in C. The single-threaded buffered/stable stream state machine,
-MT outer scheduling and flush policy, external-sequence/literals block loop,
-block decoder wrappers, and decompression result policy 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.
+Dictionary ingestion and CDict lifecycle, the CLI zstd compression leaf, and
+private CCtx, matchfinder, workspace, and codec callbacks remain in C. The
+single-threaded buffered/stable stream state machine, MT outer scheduling and
+flush policy, sequence-store and block policy, external-sequence/literals
+block loop, optional-format decompression loops, block decoder wrappers, and
+decompression result policy 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