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

rust/cli/src/lib.rs

@@ -1,5 +1,9 @@
 #[path = "../../src/benchfn.rs"]
 mod benchfn;
+#[path = "../../src/datagen.rs"]
+mod datagen;
+#[path = "../../src/lorem.rs"]
+mod lorem;
 #[path = "../../src/timefn.rs"]
 mod timefn;
 #[cfg(feature = "cli")]