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

tests/Makefile

@@ -36,6 +36,18 @@ PRGDIR  = ../programs
 PYTHON ?= python3
 TESTARTEFACT := versionsTest
 
+# The Rust implementation is linked into C compatibility tests. Keep the
+# archive a real prerequisite so a changed Rust source always relinks a test.
+CARGO ?= cargo
+RUST_DIR := ../rust
+RUST_MANIFEST := $(RUST_DIR)/Cargo.toml
+RUST_STATICLIB := $(RUST_DIR)/target/release/libzstd_rs.a
+RUST_SOURCES := $(RUST_MANIFEST) $(RUST_DIR)/Cargo.lock \
+                $(shell find $(RUST_DIR)/src -type f -name '*.rs' -print)
+
+$(RUST_STATICLIB): $(RUST_SOURCES)
+	$(CARGO) build --manifest-path $(RUST_MANIFEST) --release
+
 DEBUGFLAGS += -g -Wno-c++-compat
 CPPFLAGS   += -I$(LIB_SRCDIR) -I$(LIB_SRCDIR)/common -I$(LIB_SRCDIR)/compress -I$(LIB_SRCDIR)/legacy \
               -I$(LIB_SRCDIR)/dictBuilder -I$(LIB_SRCDIR)/deprecated -I$(PRGDIR) \
@@ -74,7 +86,6 @@ MULTITHREAD_CPP = -DZSTD_MULTITHREAD
 MULTITHREAD_LD  = -pthread
 endif
 MULTITHREAD = $(MULTITHREAD_CPP) $(MULTITHREAD_LD)
-LDFLAGS += -Wl,--whole-archive ../rust/target/release/libzstd_rs.a -Wl,--no-whole-archive
 
 VOID = /dev/null
 ZSTREAM_TESTTIME ?= -T90s
@@ -245,6 +256,15 @@ CLEAN += poolTests
 poolTests : $(PRGDIR)/util.c $(PRGDIR)/timefn.c poolTests.c $(LIB_SRCDIR)/common/pool.c $(LIB_SRCDIR)/common/threading.c $(LIB_SRCDIR)/common/zstd_common.c $(LIB_SRCDIR)/common/error_private.c
 	$(LINK.c) $(MULTITHREAD) $^ -o $@$(EXT)
 
+# These static C test executables exercise Rust replacements. The normal
+# prerequisite is also an archive input, placed after C sources by `$^`, and
+# rebuilding Rust therefore relinks rather than leaving a stale implementation.
+RUST_LINK_TARGETS := $(FULLBENCHS) fuzzer fuzzer32 zstreamtest zstreamtest32 \
+                     zstreamtest_asan zstreamtest_tsan zstreamtest_ubsan \
+                     paramgrill roundTripCrash longmatch largeDictionary \
+                     invalidDictionaries legacy decodecorpus poolTests
+$(RUST_LINK_TARGETS): $(RUST_STATICLIB)
+
 .PHONY: versionsTest
 versionsTest: clean
 	$(PYTHON) test-zstd-versions.py