build(rust): link Rust CLI generators into program tests

Keep the original C translation units as declaration-only build shims while
the Rust CLI helper archive owns datagen and lorem. Track both Rust sources in
the program and test archive prerequisites, and link the helpers-only archive
into C test binaries that still provide their own file I/O and orchestration.
This preserves the existing C test harnesses while making the migrated helper
implementations the single definitions used by the binary and tests.

Test Plan:
- make -C programs zstd V=1
- make -C tests datagen V=1
- make -C tests test-rust-lib-smoke V=1
- git diff --check

Refs: programs/datagen.c, programs/lorem.c, tests/Makefile

programs/Makefile

@@ -33,7 +33,8 @@ RUST_SOURCES := $(RUST_MANIFEST) $(RUST_DIR)/Cargo.lock \
                 $(shell find $(RUST_DIR)/src -type f -name '*.rs' -print)
 RUST_CLI_SOURCES := $(RUST_CLI_MANIFEST) $(RUST_CLI_DIR)/Cargo.lock \
                     $(RUST_CLI_DIR)/src/lib.rs $(RUST_DIR)/src/zstd_cli.rs \
-                    $(RUST_DIR)/src/timefn.rs $(RUST_DIR)/src/benchfn.rs
+                    $(RUST_DIR)/src/timefn.rs $(RUST_DIR)/src/benchfn.rs \
+                    $(RUST_DIR)/src/datagen.rs $(RUST_DIR)/src/lorem.rs
 
 # Keep Rust's HUF implementation in lockstep with libzstd.mk's C selection.
 # Forced modes may arrive as libzstd.mk variables or as direct -D flags in