build(rust): add legacy feature scaffolding

The legacy decoders (lib/legacy/zstd_v01.c .. zstd_v07.c) are next in
the Rust migration. Each of those files is a frozen snapshot of the
FSE/Huff0 entropy coders and frame logic of one historical release, so
their ports must not reuse the modern Rust entropy modules and must not
share code with each other: outputs and error codes have to stay
byte-identical to the frozen C forever. This commit installs the
build-system scaffolding so seven per-version ports can land
independently, each adding only its own module file plus a one-line
registration in rust/src/legacy/mod.rs.

Cargo grows features legacy-v01 .. legacy-v07. They are never default
features: the C build defaults differ per build system, so each build
system passes the list explicitly, derived from its own legacy
configuration:

- lib/Makefile and programs/Makefile map ZSTD_LEGACY_SUPPORT=N to the
  features for versions N..7 (0 disables legacy), mirroring the
  ZSTD_LEGACY_FILES selection in lib/libzstd.mk.
- tests/Makefile always enables all seven features because its
  ZSTDLEGACY_FILES wildcard compiles every lib/legacy/*.c regardless of
  the dispatch level.
- build/meson maps legacy_level exactly like the makefiles; build/cmake
  enables all seven whenever ZSTD_LEGACY_SUPPORT is ON because it
  always compiles all seven C files (ZSTD_LEGACY_LEVEL only selects the
  C dispatch).

Every build system also encodes the legacy selection in the Rust target
directory name (e.g. c1-d1-default-legacy5), for the same reason the
HUF mode is encoded there: a cached archive built for one configuration
must never be linked into a build expecting another. In tests/Makefile
the legacy level additionally flows into the existing HUF C-mode stamp,
so the flat C test objects (which bake -DZSTD_LEGACY_SUPPORT into the
dispatch) are rebuilt whenever the level changes. In programs/Makefile
the compress-only, decompress-only, and CLI archives keep
level-independent directories (RUST_HUF_MODE) because they are only
linked into ZSTD_LEGACY_SUPPORT=0 program variants and carry no legacy
features.

A feature whose version has not been ported yet gates nothing: the
module registration in rust/src/legacy/mod.rs is added by each port,
so enabling e.g. legacy-v05 today simply leaves that decoder in C.
This is what makes mixed C/Rust legacy levels link cleanly while the
seven ports land in any order.

Test plan:
- cd rust && cargo fmt --check && cargo clippy --all-targets
  -- -D warnings && cargo test --all-targets
- cargo clippy with --no-default-features --features
  decompression,legacy-v01 and with all seven legacy features
- make -C tests fuzzer && ./tests/fuzzer -i1 --no-big-tests
- make -C tests test-rust-lib-smoke; make -C tests test-legacy
- make -C lib libzstd.a with ZSTD_LEGACY_SUPPORT=0, 1 and default (5)
- cmake configure and meson setup (including -Dlegacy_level=1) emit the
  expected --features lists and legacy-suffixed target directories

lib/Makefile

@@ -104,6 +104,18 @@ ifneq ($(ZSTD_LIB_DECOMPRESSION),0)
 RUST_CARGO_FEATURES += $(RUST_HUF_FEATURE)
 endif
 endif
+
+# Legacy decoders follow the ZSTD_LEGACY_FILES selection exactly: level N
+# enables versions v0.N .. v0.7 (0 disables legacy).  The build directory
+# also encodes the level, so archives for different legacy levels never mix.
+RUST_LEGACY_FEATURES :=
+ifneq ($(ZSTD_LEGACY_SUPPORT), 0)
+ifeq ($(shell test $(ZSTD_LEGACY_SUPPORT) -lt 8; echo $$?), 0)
+RUST_LEGACY_FEATURES := $(addprefix legacy-v0,$(wordlist $(ZSTD_LEGACY_SUPPORT),7,1 2 3 4 5 6 7))
+endif
+endif
+RUST_CARGO_FEATURES += $(RUST_LEGACY_FEATURES)
+RUST_BUILD_CONFIG := $(RUST_BUILD_CONFIG)-legacy$(ZSTD_LEGACY_SUPPORT)
 RUST_CARGO_FEATURES := $(subst $(space),$(comma),$(strip $(RUST_CARGO_FEATURES)))
 
 RUST_TARGET ?=