docs(rust): refresh migration boundary

Update the Rust migration map after the latest compression and CLI seams
moved. The fileio summary now distinguishes Rust-owned zstd stream
orchestration from C-owned codec, diagnostics, and adaptive-policy callbacks.
The remaining-boundary summary records that dictionary dispatch and the CLI
zstd compression loop have moved, while CDict lifecycle, private dictionary
content loading, and private compressor operations remain in C.

Test Plan:
- `git diff --cached --check` -- passed.
- Documentation-only change; no additional build or test run was needed.

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rust/README.md

@@ -107,10 +107,11 @@ 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, 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,
+    asynchronous-pool adapters, metadata, zstd codec/error mapping, and
+    adaptive compression policy, while Rust owns the mixed-format
+    probe/dispatch loop, zstd stream-compression I/O 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`,
@@ -122,12 +123,13 @@ zstd ABI:
     feature, because the parser layer requires the C `fileio` backend that
     tests do not compile.
 
-Dictionary ingestion and CDict lifecycle, the CLI zstd compression leaf, and
-private CCtx, matchfinder, workspace, and codec callbacks remain in C. The
+Dictionary-ingestion dispatch, the CLI zstd compression stream loop, 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
+decompression result policy now run in Rust. CDict lifecycle, the private
+dictionary-content loader, private CCtx/matchfinder/workspace operations, and
+codec/adaptive-policy callbacks remain in C. 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.