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

rust/README.md

@@ -57,6 +57,9 @@ zstd ABI:
   - `pool` implements the bounded worker pool used by multithreaded compression.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
+- Legacy decoding
+  - `legacy` hosts one frozen module per historical format (v0.1 through
+    v0.7).  No version has been ported yet; all decoders are still C.
 - Block decompression
   - `zstd_decompress_block` decodes literal and sequence sections, maintains
     FSE/Huffman repeat state, and executes compressed-block sequences.
@@ -80,11 +83,45 @@ zstd ABI:
     Rust parser through the `ZSTD_NOBENCH`-gated bridge in `zstdcli.c`.
 
 The optimal block matcher, high-level frame compression, dictionary-building
-except suffix-array construction, legacy decoding callbacks, benchmark
-orchestration (`benchzstd`), and the CLI
-file-I/O backend are still C. They must move before the rewrite is complete.
-Keeping that boundary explicit prevents a passing hybrid build from being
-mistaken for the final all-Rust result.
+except suffix-array construction, the legacy v0.1-v0.7 decoders, benchmark
+orchestration (`benchzstd`), and the CLI file-I/O backend are still C. They
+must move before the rewrite is complete. Keeping that boundary explicit
+prevents a passing hybrid build from being mistaken for the final all-Rust
+result.
+
+## Legacy decoding
+
+Each `lib/legacy/zstd_v0N.c` file is a frozen snapshot of the entropy coders
+and frame logic of one historical release. The Rust ports in `src/legacy/`
+keep that property: every version owns its own frozen FSE/Huff0 and frame
+logic, ported line by line, and must never reuse the modern entropy modules
+or share code with other legacy versions. Outputs and error codes must be
+byte-identical to the original C files. Their only shared dependency is the
+`errors` module, matching the C files' `error_private.h` include.
+
+Cargo features `legacy-v01` .. `legacy-v07` gate the per-version modules and
+are never default features. The build systems derive the feature list from
+the C configuration:
+
+- `lib/Makefile` and `programs/Makefile` map `ZSTD_LEGACY_SUPPORT=N` to the
+  features for versions >= N (0 disables legacy), matching the
+  `ZSTD_LEGACY_FILES` selection in `lib/libzstd.mk`.
+- `tests/Makefile` always enables all seven features because the test
+  objects compile every `lib/legacy/*.c` file regardless of dispatch level.
+- `build/meson` maps `legacy_level` like the makefiles; `build/cmake`
+  enables all seven whenever `ZSTD_LEGACY_SUPPORT` is on because it always
+  compiles all seven C files.
+
+Every build system also encodes the legacy selection in the Rust target
+directory name (for example `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 that expects another.
+
+A feature whose version has not been ported yet gates nothing; the original
+C file still provides that decoder, so mixed C/Rust legacy levels link
+cleanly. Porting a version means adding `src/legacy/zstd_v0N.rs`, registering
+it in `src/legacy/mod.rs` behind its feature, and reducing
+`lib/legacy/zstd_v0N.c` to a declaration-only shim.
 
 ## Compatibility boundary