build(rust): link migrated code into native libraries

Build feature-matched Rust archives for the native Make library path. Flatten
their members into libzstd.a so static consumers resolve C-to-Rust and
Rust-to-C references in one archive, and whole-archive link shared outputs so
every migrated public ABI export remains available.

Rust compression and decompression features now mirror the C partial-library
switches. This prevents compression-only artifacts from retaining DDict
references to decompression-only C helpers.

Add a C archive smoke test that round-trips data and exercises the DDict
lifecycle through lib/libzstd.a, rather than direct test-object links.

Test Plan:
- cargo fmt, strict Clippy, cargo test --all-targets, release build
- Rust checks with compression-only, decompression-only, and no features
- full, partial, forced-HUF, and 32-bit static/shared Make library probes
- test-rust-lib-smoke, fuzzer, zstreamtest, and invalidDictionaries

Refs: rust/README.md

rust/src/lib.rs

@@ -7,14 +7,19 @@ pub mod cpu;
 pub mod debug;
 pub mod entropy_common;
 pub mod errors;
+#[cfg(feature = "compression")]
 pub mod fse_compress;
 pub mod fse_decompress;
+#[cfg(feature = "compression")]
 pub mod hist;
+#[cfg(feature = "decompression")]
 pub mod huf_decompress;
 pub mod mem;
 pub mod pool;
 pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;
+#[cfg(feature = "decompression")]
 pub mod zstd_ddict;
+#[cfg(feature = "compression")]
 pub mod zstd_presplit;