feat(decompress): move DCtx prefix copy into Rust

Move the `ZSTD_copyDCtx` prefix-copy leaf into the Rust decompression module.
Rust uses the C-projected address of `inBuff` to preserve the exact private
context cutoff, while C retains the configuration-dependent context layout
and shallow-pointer ownership semantics. Add a focused boundary test and
remove the now-unused C copy helper.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml copy_prefix -- --nocapture
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 make -B -C programs -j1 zstd
- ulimit -v 41943040 make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s

lib/decompress/zstd_decompress.c

@@ -117,7 +117,6 @@ ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
                                     ZSTD_dictLoadMethod_e dictLoadMethod,
                                     ZSTD_dictContentType_e dictContentType,
                                     ZSTD_customMem customMem);
-void ZSTD_rust_dctx_copy_prefix(ZSTD_DCtx* dstDCtx, const ZSTD_DCtx* srcDCtx);
 void ZSTD_rust_dctx_trace_begin(ZSTD_DCtx* dctx);
 void ZSTD_rust_dctx_trace_end(ZSTD_DCtx* dctx, U64 uncompressedSize,
                               U64 compressedSize, int streaming);
@@ -261,12 +260,6 @@ ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
                                      dictContentType, customMem);
 }
 
-void ZSTD_rust_dctx_copy_prefix(ZSTD_DCtx* dstDCtx, const ZSTD_DCtx* srcDCtx)
-{
-    size_t const toCopy = (size_t)((const char*)(&dstDCtx->inBuff) - (const char*)dstDCtx);
-    ZSTD_memcpy(dstDCtx, srcDCtx, toCopy);
-}
-
 void ZSTD_rust_dctx_trace_begin(ZSTD_DCtx* dctx)
 {
 #if ZSTD_TRACE