fix(build): gate CLI dictionary builder by feature

Keep dictionary-training code out of compressor-only and decompressor-only CLI
archives, where its ZDICT symbols are intentionally absent. Add a dedicated
Rust CLI archive for zstd-dictBuilder, enable the builder feature only for the
full and dictionary-builder programs, and make reduced-feature help use the
feature-independent max-level helper.

Test Plan:
- cargo test --manifest-path rust/cli/Cargo.toml --lib -- --test-threads=1 (158 passed)
- cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,dict-builder --lib -- --test-threads=1 (124 passed)
- cargo clippy --manifest-path rust/cli/Cargo.toml --lib -- -D warnings
- cargo clippy --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,dict-builder --lib -- -D warnings
- make -C programs -j2 zstd zstd-small zstd-frugal zstd-decompress zstd-compress zstd-dictBuilder
- git diff --check

rust/cli/Cargo.toml

@@ -14,6 +14,7 @@ default = ["cli", "compression", "decompression", "benchmark"]
 cli = []
 compression = []
 decompression = []
+dict-builder = ["compression"]
 helpers = []
 # The integrated benchmark archive is optional so zstd-small and zstd-frugal
 # can omit every benchmark implementation and its timing/data-generator