docs(rust): refresh dictionary migration boundary

Document that the Rust dictionary-builder modules now own sample analysis,
training, and dictionary assembly, with the original C files retained only as
ABI/build shims. Remove the stale statement that only suffix-array
construction has migrated and keep the remaining C boundary focused on
compression orchestration, benchmark reporting, and format-specific CLI I/O.

Test Plan:
- git diff --cached --check
- make -C tests -j2 test-zstd (passed before this documentation-only commit)

rust/README.md

@@ -57,8 +57,10 @@ zstd ABI:
     maintenance, sequence generation, and sequence consumption.
 - Dictionary building
   - `divsufsort` constructs the suffix array that drives the legacy `ZDICT`
-    trainer (`ZDICT_trainFromBuffer_legacy`).  The sample analysis and
-    dictionary assembly in `zdict.c`, `cover.c`, and `fastcover.c` remain C.
+    trainer (`ZDICT_trainFromBuffer_legacy`); `dict_builder_zdict`,
+    `dict_builder_cover`, and `dict_builder_fastcover` own the sample analysis,
+    training, and dictionary assembly.  The corresponding C translation units
+    are declaration-only ABI shims.
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.
@@ -94,11 +96,10 @@ zstd ABI:
     Rust parser through the `ZSTD_NOBENCH`-gated bridge in `zstdcli.c`.
 
 The optimal block matcher, most high-level frame-compression orchestration,
-dictionary-building except suffix-array construction, benchmark orchestration
-and reporting (`benchzstd`), and several format-specific CLI I/O paths are
-still C. They 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.
+benchmark orchestration and reporting (`benchzstd`), and several
+format-specific CLI I/O paths are still C. They 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