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.

build/cmake/lib/CMakeLists.txt

@@ -142,6 +142,7 @@ endif()
 set(_zstd_rust_features)
 set(_zstd_rust_compression 0)
 set(_zstd_rust_decompression 0)
+set(_zstd_rust_dictbuilder 0)
 if(ZSTD_BUILD_COMPRESSION)
     list(APPEND _zstd_rust_features compression)
     set(_zstd_rust_compression 1)
@@ -150,6 +151,10 @@ if(ZSTD_BUILD_DECOMPRESSION)
     list(APPEND _zstd_rust_features decompression)
     set(_zstd_rust_decompression 1)
 endif()
+if(ZSTD_BUILD_DICTBUILDER)
+    list(APPEND _zstd_rust_features dict-builder)
+    set(_zstd_rust_dictbuilder 1)
+endif()
 
 set(_zstd_rust_huf_mode default)
 if(_zstd_huf_force_x1)
@@ -190,7 +195,7 @@ if(_zstd_rust_features)
 endif()
 
 set(_zstd_rust_build_config
-    "c${_zstd_rust_compression}-d${_zstd_rust_decompression}-${_zstd_rust_huf_mode}")
+    "c${_zstd_rust_compression}-d${_zstd_rust_decompression}-b${_zstd_rust_dictbuilder}-${_zstd_rust_huf_mode}")
 set(ZSTD_RUST_MANIFEST "${ZSTD_SOURCE_DIR}/rust/Cargo.toml")
 set(ZSTD_RUST_TARGET_DIR
     "${CMAKE_CURRENT_BINARY_DIR}/rust-target/${_zstd_rust_build_config}")

build/meson/lib/meson.build

@@ -73,7 +73,9 @@ if rust_huf_force_x1 and rust_huf_force_x2
   error('HUF_FORCE_DECOMPRESS_X1 and HUF_FORCE_DECOMPRESS_X2 are mutually exclusive')
 endif
 
-rust_features = ['compression', 'decompression']
+# Meson always compiles the dictBuilder sources above, so the Rust archive
+# must always carry the matching dict-builder module set.
+rust_features = ['compression', 'decompression', 'dict-builder']
 rust_huf_mode = 'default'
 rust_huf_c_args = []
 if rust_huf_force_x1
@@ -96,7 +98,7 @@ if rust_target == ''
   endif
 endif
 
-rust_build_config = 'c1-d1-' + rust_huf_mode
+rust_build_config = 'c1-d1-b1-' + rust_huf_mode
 rust_target_dir = join_paths(meson.current_build_dir(), 'rust-target', rust_build_config)
 is_msvc = cc_id == compiler_msvc or cc_id == 'clang-cl'
 rust_staticlib_name = is_msvc ? 'zstd_rs.lib' : 'libzstd_rs.a'