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/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
-//! implementation.  It reuses the mature C `fileio` layer through its narrow
-//! public-in-the-programs-tree ABI: file opening, safe replacement, sparse
-//! writes, dictionary loading, streaming, and metadata preservation remain in
-//! `programs/fileio.c` for this first migration step.
+//! This is the Rust parser and dispatch layer plus selected file-I/O leaves,
+//! not yet a fully independent file-I/O implementation.  It shares the
+//! mature C `fileio` layer through a narrow public-in-the-programs-tree ABI:
+//! Rust owns preference policy, filename decisions, file opening, dictionary
+//! 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