docs(rust): refresh migration boundary

Update the Rust rewrite documentation to match the current implementation
boundary.  The legacy decoder set is complete, and Rust now owns several
CLI file-I/O policy and resource leaves while C retains format dispatch,
metadata, and remaining orchestration.  Keep the stated boundary explicit so
passing hybrid tests do not imply that the full rewrite is finished.

Test Plan:
- git diff --cached --check

rust/README.md

@@ -66,8 +66,9 @@ zstd ABI:
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 - Legacy decoding
   - `legacy` hosts one frozen module per historical format; `legacy::zstd_v01`
-    ports the self-contained v0.1 decoder.  Versions v0.2 through v0.7 are
+    through `legacy::zstd_v07` port the seven self-contained historical
-    still C.
+    decoders.  Their original C translation units remain declaration-only
+    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.
@@ -78,8 +79,10 @@ 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` backend still owns file opening, safe replacement, sparse writes,
+    `fileio` layer still owns format-specific dispatch, metadata, and the
-    metadata, and streaming I/O.
+    remaining C streaming/orchestration paths.  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`,
     and `benchfn` owns the benchmark run/timing loop (`BMK_benchFunction`,
     `BMK_benchTimedFn`) used by the CLI benchmark mode and by C test tools.
@@ -90,12 +93,12 @@ zstd ABI:
     orchestration and reporting (`benchzstd.c`) remain C, reached from the
     Rust parser through the `ZSTD_NOBENCH`-gated bridge in `zstdcli.c`.
 
-The optimal block matcher, high-level frame compression, dictionary-building
+The optimal block matcher, most high-level frame-compression orchestration,
-except suffix-array construction, the legacy v0.2-v0.7 decoders, benchmark
+dictionary-building except suffix-array construction, benchmark orchestration
-orchestration (`benchzstd`), and the CLI file-I/O backend are still C. They
+and reporting (`benchzstd`), and several format-specific CLI I/O paths are
-must move before the rewrite is complete. Keeping that boundary explicit
+still C. They must move before the rewrite is complete. Keeping that boundary
-prevents a passing hybrid build from being mistaken for the final all-Rust
+explicit prevents a passing hybrid build from being mistaken for the final
-result.
+all-Rust result.
 
 ## Legacy decoding
 
@@ -108,8 +111,8 @@ byte-identical to the original C files. Their only shared dependency is the
 `errors` module, matching the C files' `error_private.h` include.
 
 Cargo features `legacy-v01` .. `legacy-v07` gate the per-version modules and
-are never default features. The build systems derive the feature list from
+are never default features. All seven modules are now available; the build
-the C configuration:
+systems derive the enabled feature list from the C configuration:
 
 - `lib/Makefile` and `programs/Makefile` map `ZSTD_LEGACY_SUPPORT=N` to the
   features for versions >= N (0 disables legacy), matching the
@@ -125,13 +128,11 @@ directory name (for example `c1-d1-default-legacy5`), for the same reason the
 HUF mode is encoded there: a cached archive built for one configuration must
 never be linked into a build that expects another.
 
-A feature whose version has not been ported yet gates nothing; the original
+Each port adds `src/legacy/zstd_v0N.rs`, registers it in `src/legacy/mod.rs`
-C file still provides that decoder, so mixed C/Rust legacy levels link
+behind its feature, and reduces `lib/legacy/zstd_v0N.c` to a
-cleanly. Porting a version means adding `src/legacy/zstd_v0N.rs`, registering
+declaration-only shim. For v0.1 the streaming `ZSTDv01_Dctx` state lives
-it in `src/legacy/mod.rs` behind its feature, and reducing
+entirely in Rust: C code only ever holds an opaque pointer, so the C-side
-`lib/legacy/zstd_v0N.c` to a declaration-only shim. For v0.1 the streaming
+struct definition is gone.
-`ZSTDv01_Dctx` state lives entirely in Rust: C code only ever holds an opaque
-pointer, so the C-side struct definition is gone.
 
 ## Compatibility boundary
 

rust/src/zstd_cli.rs

@@ -4,11 +4,13 @@
 
 //! Rust command-line frontend for zstd.
 //!
-//! This is intentionally a parser and dispatch layer, not a second file I/O
+//! This is the Rust parser and dispatch layer plus selected file-I/O leaves,
-//! implementation.  It reuses the mature C `fileio` layer through its narrow
+//! not yet a fully independent file-I/O implementation.  It shares the
-//! public-in-the-programs-tree ABI: file opening, safe replacement, sparse
+//! mature C `fileio` layer through a narrow public-in-the-programs-tree ABI:
-//! writes, dictionary loading, streaming, and metadata preservation remain in
+//! Rust owns preference policy, filename decisions, file opening, dictionary
-//! `programs/fileio.c` for this first migration step.
+//! loading, asynchronous pools, and pass-through copying, while C still owns
+//! format-specific dispatch, metadata preservation, and remaining streaming
+//! orchestration.
 //!
 //! Benchmark mode (`-b`) parses and dispatches through the Rust CLI archive.
 //! The bridge preserves the C CLI's argument normalization and uses the