feat(rust): port legacy v0.1 decoder

Port lib/legacy/zstd_v01.c (the frozen zstd v0.1 decoder) to
rust/src/legacy/zstd_v01.rs as the first legacy-format port on the new
scaffolding, and reduce the C file to a declaration-only shim that
keeps its header includes for configuration and platform preprocessor
behavior.

Frozen-decoder policy: zstd_v01.c embeds its own v0.1-era FSE and
Huff0 snapshot, distinct from every other release. The Rust port is a
line-by-line translation with the same table layouts (FSE_DTable as a
u32 header word plus packed newState/symbol/nbBits entries, the Huff0
u16 DTable with byte/nbBits pairs), the same arithmetic including
wrap-around and pointer-comparison quirks (e.g. the offset-vs-base
address check in ZSTD_execSequence), the same internal FSE error space
(size_t)-1..-7, and the same public ZSTD error codes. It reuses no
modern Rust entropy module; its only crate dependency is `errors`,
matching the C file's error_private.h include. The 32-bit-only reload
points are kept as compile-time conditions on usize::BITS.

Symbol takeover boundary: all nine ZSTDv01_* entry points from
zstd_v01.h now come from Rust as context-free #[no_mangle] extern "C"
functions (isError, decompress, decompressDCtx,
findFrameSizeInfoLegacy, createDCtx, freeDCtx, resetDCtx,
nextSrcSizeToDecompress, decompressContinue). zstd_legacy.h only uses
the first four for v0.1; streaming for v0.1-v0.3 intentionally returns
version_unsupported there, unchanged. The ZSTDv01_Dctx struct
definition moves entirely into Rust: C code only ever holds an opaque
pointer (zstd_v01.h forward-declares the type), and the context is
malloc/free-allocated exactly like the C version so create/free may
pair across the language boundary.

Byte-identity verification against the pristine pre-migration C build
(f8745da6, pure C, ZSTD_LEGACY_SUPPORT=1):

- Real v0.1 frames were generated by building the v0.1.0 git tag and
  compressing text, random, and 426 KB multi-block inputs. A one-shot
  ZSTD_decompress harness linked once against the pristine C libzstd.a
  and once against the Rust-backed libzstd.a produced bit-identical
  outputs for all frames.
- A direct ZSTDv01_* probe (one-shot decode, dst-too-small, truncated
  input, bad magic, findFrameSizeInfoLegacy, and the streaming
  continue loop) printed identical results, including exact error
  codes (-70 dstSize_tooSmall, -72 srcSize_wrong, -10 prefix_unknown)
  and identical dBound values.
- zstd -l -v on v0.1 files matches the pristine binary; CLI streaming
  decode of v0.1 fails with the same "Version not supported" in both,
  by design of zstd_legacy.h.

Unit tests embed three v0.1.0-generated fixtures (entropy-coded,
raw-block, and four-block frames) plus the truncation, bad-magic,
small-destination, and streaming-API cases, all asserting the exact C
error codes above. Note that `make -C tests test-legacy` only covers
v0.4+ frames, so the embedded fixtures and the harness comparison are
the actual v0.1 coverage.

Test plan:
- cd rust && cargo fmt --check && cargo clippy --all-targets
  --features legacy-v01 -- -D warnings && cargo test --all-targets
  --features legacy-v01 (127 tests, 9 for v0.1)
- cargo clippy/test --no-default-features --features
  decompression,legacy-v01 (module builds standalone)
- make -C tests fuzzer && ./tests/fuzzer -i1 --no-big-tests, also with
  ZSTD_LEGACY_SUPPORT=1 (mixed Rust v0.1 + C v0.2-0.7 link)
- make -C tests test-rust-lib-smoke && make -C tests test-legacy
- make -C programs zstd (default and ZSTD_LEGACY_SUPPORT=1); nm shows
  the nine ZSTDv01_* symbols provided by Rust at level 1
- make -C lib libzstd.a ZSTD_LEGACY_SUPPORT=0 (no legacy symbols) and
  meson -Dlegacy_level=1 shared library exporting all nine

rust/README.md

@@ -48,8 +48,9 @@ zstd ABI:
 - 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.
+  - `legacy` hosts one frozen module per historical format; `legacy::zstd_v01`
+    ports the self-contained v0.1 decoder.  Versions v0.2 through v0.7 are
+    still C.
 - Block decompression
   - `zstd_decompress_block` decodes literal and sequence sections, maintains
     FSE/Huffman repeat state, and executes compressed-block sequences.
@@ -64,7 +65,7 @@ zstd ABI:
     metadata, and streaming I/O.
 
 The optimal block matcher, high-level frame compression, dictionary-building,
-the legacy v0.1-v0.7 decoders, and the CLI file-I/O backend are still C. They
+the legacy v0.2-v0.7 decoders, 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.
@@ -101,7 +102,9 @@ 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.
+`lib/legacy/zstd_v0N.c` to a declaration-only shim. For v0.1 the streaming
+`ZSTDv01_Dctx` state lives entirely in Rust: C code only ever holds an opaque
+pointer, so the C-side struct definition is gone.
 
 ## Compatibility boundary