build(meson): link migrated Rust code into native libraries

Build a configuration-matched Cargo archive from Meson and flatten its object
members into static libzstd.  This gives C consumers one archive even though
the migrated Rust objects and remaining C code refer to each other.

Shared libraries whole-archive the Cargo output to retain Rust-only ABI
exports.  The implementation supports Meson 0.50's generated-archive linking
rules, matching HUF mode and 32-bit configuration, and documents cross-build
target and archiver selection.

Test Plan:
- fresh Meson both-build smoke consumers and invalidDictionaries
- modern static/shared/both, forced-HUF, i686, install, and fuzzer paths
- real Meson 0.50 static and shared smoke builds
- cargo clippy, cargo clippy --benches, cargo clippy --tests, and nightly fmt

Refs: build/meson/README.md archive and cross-build documentation

build/meson/README.md

@@ -12,6 +12,12 @@ by Dima Krasner \<dima@dimakrasner.com\>.
 It outputs one `libzstd`, either shared or static, depending on
 `default_library` option.
 
+The migrated compatibility modules are built with Cargo as part of every
+Meson library build.  Static builds flatten the Cargo object members into
+`libzstd.a`, so external C consumers link one archive.  Shared builds use a
+whole-archive link for the Cargo archive, retaining Rust-only public ABI
+exports.
+
 ## How to build
 
 `cd` to this meson directory (`build/meson`)
@@ -36,3 +42,14 @@ meson configure
 ```
 
 See [man meson(1)](https://manpages.debian.org/testing/meson/meson.1.en.html).
+
+For a cross build, set the Cargo target explicitly, for example:
+
+```sh
+meson setup -Dzstd_rust_target=aarch64-unknown-linux-gnu builddir
+```
+
+On 32-bit Linux, the `i686-unknown-linux-gnu` target is selected
+automatically.  Set `-Dzstd_rust_archiver=...` when the C cross toolchain needs
+a non-default archiver.  The `huf_force_decompress_x1` and
+`huf_force_decompress_x2` options select matching C and Rust decoder modes.