docs(rust): refresh decoder migration boundary

The component map still described decoder context allocation as a C-owned
responsibility after the allocation and custom-memory bridges moved into Rust.
Update the boundary description to distinguish the C-defined context layout and
configuration-sensitive platform, legacy, and trace leaves from Rust-owned
storage allocation and allocator dispatch.

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

rust/README.md

@@ -76,7 +76,9 @@ zstd ABI:
     FSE/Huffman repeat state, and executes compressed-block sequences.
   - `zstd_decompress` owns the public decompression context, one-shot,
     dictionary, parameter, and streaming state machines.  Its C shim retains
-    configuration-dependent context allocation plus legacy and trace leaves.
+    the configuration-dependent context layout and platform details, plus
+    legacy and trace leaves; Rust owns decoder storage allocation and custom
+    memory dispatch.
 - Command-line frontend
   - `zstd_cli` owns the Rust parser, safety policy, and dispatch.  It is built
     by the separate `cli/` static-library package only for program archives,