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/tests/meson.build

@@ -35,13 +35,12 @@ testcommon_sources = [join_paths(zstd_rootdir, 'programs/datagen.c'),
   join_paths(zstd_rootdir, 'programs/benchfn.c'),
   join_paths(zstd_rootdir, 'programs/benchzstd.c')]
 
-testcommon = static_library('testcommon',
-  testcommon_sources,
-  # needed due to use of private symbol + -fvisibility=hidden
-  link_with: libzstd_static)
+# The final executable carries libzstd: static archives cannot consume a
+# custom-generated archive directly on Meson 0.50.
+testcommon = static_library('testcommon', testcommon_sources)
 
 testcommon_dep = declare_dependency(link_with: testcommon,
-  dependencies: libzstd_deps,
+  dependencies: [libzstd_deps, libzstd_static_dep],
   include_directories: libzstd_includes)
 
 datagen_sources = [join_paths(zstd_rootdir, 'tests/datagencli.c'),
@@ -102,6 +101,22 @@ invalidDictionaries = executable('invalidDictionaries',
   dependencies: [ libzstd_dep ],
   install: false)
 
+# A native consumer must need only libzstd.a.  This exercises the flattened
+# C/Rust archive rather than relying on Cargo's archive order at link time.
+rustLibSmokeStatic = executable('rustLibSmokeStatic',
+  join_paths(zstd_rootdir, 'tests/rustLibSmoke.c'),
+  dependencies: [libzstd_static_dep],
+  install: false)
+test('test-rustLibSmokeStatic', rustLibSmokeStatic)
+
+if default_library_type != 'static'
+  rustLibSmokeShared = executable('rustLibSmokeShared',
+    join_paths(zstd_rootdir, 'tests/rustLibSmoke.c'),
+    dependencies: [libzstd_dep],
+    install: false)
+  test('test-rustLibSmokeShared', rustLibSmokeShared)
+endif
+
 if 0 < legacy_level and legacy_level <= 4
   legacy_sources = [join_paths(zstd_rootdir, 'tests/legacy.c')]
   legacy = executable('legacy',