build(rust): rebuild the archive for C test and CLI links

Make Rust sources normal prerequisites of native C test executables and CLI
builds. A changed Rust module now rebuilds libzstd_rs.a and relinks the target,
instead of relying on a manually prepared archive that can silently be stale.

The archive is linked after C objects, allowing migrated C shims to resolve
their Rust ABI symbols while remaining compatible with the existing makefile
flows. The migration guide now documents this behavior.

Test Plan:
- make -B -C tests fuzzer
- ./tests/fuzzer -i1 --no-big-tests
- make -B -C programs zstd
- ./programs/zstd --version

Refs: rust/README.md

rust/README.md

@@ -39,9 +39,10 @@ source file whose implementation has moved to Rust remains in the original
 makefile source list as a small shim so header configuration and platform
 preprocessor behavior stay available during the transition.
 
-The original test makefile links `target/release/libzstd_rs.a` with whole-archive
-semantics. Rebuild the Rust archive before running C tests so the executable
-does not use a stale implementation.
+The test and program makefiles make `target/release/libzstd_rs.a` a normal
+link prerequisite. They rebuild the Rust archive and relink the executable
+when a Rust source changes, so original C tests do not accidentally use a stale
+implementation.
 
 ## Validation