build(rust): add dict-builder cargo feature dimension

The dictionary-builder sources (lib/dictBuilder) are about to start moving
to Rust, beginning with divsufsort. The Rust crate previously only modeled
the compression/decompression module split plus the forced-HUF decoder
modes, so no build could express "this C configuration includes (or
excludes) dictBuilder" to Cargo. Without that, a Rust archive could carry
dictBuilder modules into a build whose C side disabled them, or worse,
omit a migrated implementation from a build whose C shims require it.

Add a `dict-builder` cargo feature and thread it through every build that
consumes the Rust static archive, mirroring exactly how each build system
already gates the dictBuilder C sources:

- rust/Cargo.toml: new `dict-builder` feature, included in the default
  set because the C library builds dictBuilder by default
  (ZSTD_LIB_DICTBUILDER ?= 1). The feature is empty until the first
  dictBuilder module lands.
- lib/Makefile: RUST_CARGO_FEATURES gains dict-builder when
  ZSTD_LIB_DICTBUILDER is enabled, following the existing
  ZSTD_LIB_COMPRESSION/ZSTD_LIB_DECOMPRESSION pattern. The archive
  directory naming grows a matching `b<0|1>` dimension
  (c1-d1-b1-default etc.) so differently configured archives never
  collide; the repeated config prefix is factored into
  RUST_MODULE_CONFIG.
- programs/Makefile: the full-featured archives now request
  compression,decompression,dict-builder (equal to the default set, so
  the target directory stays shared with tests). The partial-library
  variants gain the `b0` name dimension, and zstd-dictBuilder gets its
  own lib-c1-d0-b1 archive because it compiles the dictBuilder C sources
  without decompression; it previously shared the compression-only
  archive, which will lack the migrated dictBuilder symbols.
- tests/Makefile: no flag change needed since tests use the crate default
  feature set; a comment now records that dict-builder arrives that way.
- build/cmake/lib/CMakeLists.txt: ZSTD_BUILD_DICTBUILDER now adds the
  dict-builder feature and a `b<0|1>` component in the Rust build-config
  directory name, in lockstep with the DictBuilderSources gating.
- build/meson/lib/meson.build: meson compiles the dictBuilder sources
  unconditionally, so the feature list and config name gain dict-builder
  unconditionally (c1-d1-b1-<huf-mode>).

The `dict-builder` feature deliberately does not imply `compression`.
lib/Makefile forces ZSTD_LIB_DICTBUILDER=0 when compression is disabled,
but CMake does not couple the two options, so encoding the C-side
constraint in Cargo would make the Rust archive diverge from the C source
list in that (already unsupported) CMake configuration.

Test plan:
- cd rust && cargo build --release
- cargo build --release --no-default-features \
    --features compression,decompression
- cargo build --release --no-default-features \
    --features compression,dict-builder
- Full validation (fuzzer, smoke tests, dictionary byte-identity) runs
  with the follow-up commit that ports divsufsort onto this scaffolding.

rust/Cargo.toml

@@ -7,9 +7,10 @@ edition = "2021"
 crate-type = ["staticlib"]
 
 [features]
-default = ["compression", "decompression"]
+default = ["compression", "decompression", "dict-builder"]
 compression = []
 decompression = []
+dict-builder = []
 huf-force-decompress-x1 = []
 huf-force-decompress-x2 = []
 

rust/README.md

@@ -74,8 +74,9 @@ makefile source list as a small shim so header configuration and platform
 preprocessor behavior stay available during the transition.
 
 The library, test, and program makefiles select an archive directory for the
-active C configuration: enabled compression/decompression modules, default or
-forced HUF X1/X2, and the matching Rust target for 32-bit C binaries. The
+active C configuration: enabled compression/decompression/dictionary-builder
+modules, default or forced HUF X1/X2, and the matching Rust target for 32-bit
+C binaries. The
 native static archive flattens Rust object members rather than nesting a Rust
 archive, while the native shared library retains all migrated Rust exports.
 When the HUF mode changes, the test and program paths also rebuild cached C