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.

programs/Makefile

@@ -71,7 +71,7 @@ RUST_TARGET_32 ?= i686-unknown-linux-gnu
 RUST_STATICLIB_32 := $(RUST_TARGET_DIR)/$(RUST_TARGET_32)/release/libzstd_rs.a
 RUST_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
                     --target-dir $(RUST_TARGET_DIR) --no-default-features
-RUST_CARGO_FLAGS += --features compression,decompression
+RUST_CARGO_FLAGS += --features compression,decompression,dict-builder
 ifneq ($(RUST_HUF_FEATURE),)
 RUST_CARGO_FLAGS += --features $(RUST_HUF_FEATURE)
 endif
@@ -96,7 +96,7 @@ $(RUST_CLI_STATICLIB): $(RUST_CLI_SOURCES)
 $(RUST_CLI_STATICLIB_32): $(RUST_CLI_SOURCES)
 	$(CARGO) build $(RUST_CLI_CARGO_FLAGS) --target $(RUST_TARGET_32)
 
-RUST_DECOMPRESS_BUILD_CONFIG := lib-c0-d1-$(RUST_BUILD_CONFIG)
+RUST_DECOMPRESS_BUILD_CONFIG := lib-c0-d1-b0-$(RUST_BUILD_CONFIG)
 RUST_DECOMPRESS_TARGET_DIR := $(RUST_DIR)/target/$(RUST_DECOMPRESS_BUILD_CONFIG)
 RUST_DECOMPRESS_STATICLIB := $(RUST_DECOMPRESS_TARGET_DIR)/release/libzstd_rs.a
 RUST_DECOMPRESS_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
@@ -119,7 +119,7 @@ RUST_DECOMPRESS_CLI_CARGO_FLAGS := --manifest-path $(RUST_CLI_MANIFEST) --releas
 $(RUST_DECOMPRESS_CLI_STATICLIB): $(RUST_CLI_SOURCES)
 	$(CARGO) build $(RUST_DECOMPRESS_CLI_CARGO_FLAGS)
 
-RUST_COMPRESS_BUILD_CONFIG := lib-c1-d0-$(RUST_BUILD_CONFIG)
+RUST_COMPRESS_BUILD_CONFIG := lib-c1-d0-b0-$(RUST_BUILD_CONFIG)
 RUST_COMPRESS_TARGET_DIR := $(RUST_DIR)/target/$(RUST_COMPRESS_BUILD_CONFIG)
 RUST_COMPRESS_STATICLIB := $(RUST_COMPRESS_TARGET_DIR)/release/libzstd_rs.a
 RUST_COMPRESS_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
@@ -129,6 +129,17 @@ RUST_COMPRESS_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
 $(RUST_COMPRESS_STATICLIB): $(RUST_SOURCES)
 	$(CARGO) build $(RUST_COMPRESS_CARGO_FLAGS)
 
+RUST_DICTBUILDER_BUILD_CONFIG := lib-c1-d0-b1-$(RUST_BUILD_CONFIG)
+RUST_DICTBUILDER_TARGET_DIR := $(RUST_DIR)/target/$(RUST_DICTBUILDER_BUILD_CONFIG)
+RUST_DICTBUILDER_STATICLIB := $(RUST_DICTBUILDER_TARGET_DIR)/release/libzstd_rs.a
+RUST_DICTBUILDER_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
+                                 --target-dir $(RUST_DICTBUILDER_TARGET_DIR) \
+                                 --no-default-features \
+                                 --features compression,dict-builder
+
+$(RUST_DICTBUILDER_STATICLIB): $(RUST_SOURCES)
+	$(CARGO) build $(RUST_DICTBUILDER_CARGO_FLAGS)
+
 RUST_COMPRESS_CLI_BUILD_CONFIG := cli-c1-d0-$(RUST_BUILD_CONFIG)
 RUST_COMPRESS_CLI_TARGET_DIR := $(RUST_DIR)/target/$(RUST_COMPRESS_CLI_BUILD_CONFIG)
 RUST_COMPRESS_CLI_STATICLIB := $(RUST_COMPRESS_CLI_TARGET_DIR)/release/libzstd_cli_rs.a
@@ -412,7 +423,7 @@ zstd-compress: $(ZSTDLIB_COMMON_SRC) $(ZSTDLIB_COMPRESS_SRC) zstdcli.c util.c ti
 
 ## zstd-dictBuilder: executable supporting dictionary creation and compression (only)
 CLEAN += zstd-dictBuilder
-zstd-dictBuilder: $(ZSTDLIB_COMMON_SRC) $(ZSTDLIB_COMPRESS_SRC) $(ZDICT_SRC) zstdcli.c util.c timefn.c fileio.c fileio_asyncio.c dibio.c $(RUST_COMPRESS_STATICLIB) $(RUST_COMPRESS_CLI_STATICLIB)
+zstd-dictBuilder: $(ZSTDLIB_COMMON_SRC) $(ZSTDLIB_COMPRESS_SRC) $(ZDICT_SRC) zstdcli.c util.c timefn.c fileio.c fileio_asyncio.c dibio.c $(RUST_DICTBUILDER_STATICLIB) $(RUST_COMPRESS_CLI_STATICLIB)
 	$(CC) $(FLAGS) -DZSTD_NOBENCH -DZSTD_NODECOMPRESS -DZSTD_NOTRACE $^ -o $@$(EXT)
 
 RUST_DIRECT_LINK_TARGETS := zstd32 zstd-nolegacy zstd-small zstd-frugal \