fix(build): rebuild C outputs on HUF mode changes

Mode-specific Rust archives exposed the test makefiles' flat C-object cache:
switching from default to forced HUF mode could relink stale default C objects
with a newly built forced Rust archive. That hybrid has incompatible decoder
table expectations and can corrupt dictionary decompression.

Track each makefile's effective HUF mode with an empty archive stamp. It is a
safe normal linker prerequisite, and every mode transition advances its mtime
so cached C objects and direct-source binaries rebuild while unchanged modes
remain incremental.

Test Plan:
- cargo clippy; cargo clippy --benches; cargo clippy --tests
- cargo +nightly fmt, then repeat the Clippy checks
- cargo test --all-targets
- Default -> forced-X1 -> default fuzzer builds without -B, each running
  ./fuzzer -s5346 -i1 --no-big-tests
- Equivalent zstd-small default/forced/default rebuild and --version checks

Refs: rust/README.md
Fixes: d89ebb31

tests/Makefile

@@ -91,6 +91,25 @@ $(RUST_STATICLIB): $(RUST_SOURCES)
 $(RUST_STATICLIB_32): $(RUST_SOURCES)
 	$(CARGO) build $(RUST_CARGO_FLAGS) --target $(RUST_TARGET_32)
 
+# These test objects have flat filenames, unlike the configuration-hashed
+# program objects. Track the HUF mode separately so a C object set compiled
+# for one decoder is never relinked with a Rust archive for another decoder.
+# The stamp is an empty archive, so normal linker `$^` recipes can consume it.
+RUST_HUF_C_MODE_FILE := $(RUST_DIR)/target/.huf-c-mode-tests
+RUST_HUF_C_MODE_STAMP := $(RUST_DIR)/target/.huf-c-mode-tests.a
+.PHONY: FORCE_RUST_HUF_C_MODE
+FORCE_RUST_HUF_C_MODE:
+$(RUST_HUF_C_MODE_STAMP): FORCE_RUST_HUF_C_MODE
+	@mkdir -p $(@D)
+	@if test ! -r $(RUST_HUF_C_MODE_FILE) \
+		|| test "$$(cat $(RUST_HUF_C_MODE_FILE))" != "$(RUST_BUILD_CONFIG)" \
+		|| test ! -f $@; then \
+		printf '%s\n' "$(RUST_BUILD_CONFIG)" > $(RUST_HUF_C_MODE_FILE); \
+		$(AR) rcs $@; \
+		touch $@; \
+	fi
+CLEAN += $(RUST_HUF_C_MODE_FILE) $(RUST_HUF_C_MODE_STAMP)
+
 DEBUGFLAGS += -g -Wno-c++-compat
 CPPFLAGS   += -I$(LIB_SRCDIR) -I$(LIB_SRCDIR)/common -I$(LIB_SRCDIR)/compress -I$(LIB_SRCDIR)/legacy \
               -I$(LIB_SRCDIR)/dictBuilder -I$(LIB_SRCDIR)/deprecated -I$(PRGDIR) \
@@ -118,6 +137,8 @@ ZSTDMT_OBJ4 := $(subst $(LIB_SRCDIR)/legacy/,zstdmt_l_,$(ZSTDMT_OBJ3))
 ZSTDMT_OBJ5 := $(ZSTDMT_OBJ4:.c=.o)
 ZSTDMT_OBJECTS := $(ZSTDMT_OBJ5:.S=.o)
 
+$(ZSTD_OBJECTS) $(ZSTDMT_OBJECTS): $(RUST_HUF_C_MODE_STAMP)
+
 # Define *.exe as extension for Windows systems
 ifneq (,$(filter Windows%,$(OS)))
 EXT =.exe
@@ -311,6 +332,8 @@ $(RUST_LINK_TARGETS): $(RUST_STATICLIB)
 RUST_LINK_TARGETS_32 := fullbench32 fuzzer32 zstreamtest32
 $(RUST_LINK_TARGETS_32): $(RUST_STATICLIB_32)
 
+$(RUST_LINK_TARGETS) $(RUST_LINK_TARGETS_32): $(RUST_HUF_C_MODE_STAMP)
+
 .PHONY: versionsTest
 versionsTest: clean
 	$(PYTHON) test-zstd-versions.py