build(rust): select archives by target and HUF mode

Derive the Rust static-library configuration from the effective C HUF decoder
flags and place each mode in its own Cargo target directory. This prevents a
forced X1/X2 C build from silently reusing the default Rust archive.

The test and program makefiles now also build and link an i686 Rust archive
for their 32-bit targets, including zstd32. zstd-small retains its existing
default configuration while preserving an explicitly requested HUF mode.

Test Plan:
- cargo clippy; cargo clippy --benches; cargo clippy --tests
- cargo +nightly fmt, then repeat the Clippy checks
- cargo test --all-targets for default, forced X1, and forced X2 modes
- cargo build --release --target i686-unknown-linux-gnu for each mode
- make -n checks for default, forced, direct -D, and 32-bit test/CLI targets

Refs: rust/README.md

rust/Cargo.toml

@@ -6,5 +6,10 @@ edition = "2021"
 [lib]
 crate-type = ["staticlib"]
 
+[features]
+default = []
+huf-force-decompress-x1 = []
+huf-force-decompress-x2 = []
+
 [dependencies]
 libc = "0.2"

rust/README.md

@@ -41,10 +41,11 @@ source file whose implementation has moved to Rust remains in the original
 makefile source list as a small shim so header configuration and platform
 preprocessor behavior stay available during the transition.
 
-The test and program makefiles make `target/release/libzstd_rs.a` a normal
-link prerequisite. They rebuild the Rust archive and relink the executable
-when a Rust source changes, so original C tests do not accidentally use a stale
-implementation.
+The test and program makefiles select an archive directory for the active C
+configuration: default, forced HUF X1/X2, and the matching Rust target for
+32-bit C binaries. They rebuild the selected archive and relink the executable
+when a Rust source changes, so original C tests do not accidentally use a
+stale or configuration-incompatible implementation.
 
 ## Validation