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

rust/src/zstd_decompress.rs

@@ -326,7 +326,6 @@ unsafe extern "C" {
         streaming: c_int,
     );
     fn ZSTD_rust_dctx_trace_begin(dctx: *mut ZSTD_DCtx);
-    fn ZSTD_rust_dctx_copy_prefix(dst: *mut ZSTD_DCtx, src: *const ZSTD_DCtx);
     #[cfg(all(
         not(test),
         any(
@@ -367,6 +366,12 @@ unsafe fn dctx_view(dctx: *mut ZSTD_DCtx) -> ZSTD_rustDctxView {
     unsafe { view.assume_init() }
 }
 
+#[inline]
+unsafe fn copy_dctx_prefix(dst: *mut u8, src: *const u8, in_buff: *const c_void) {
+    let to_copy = (in_buff as usize).wrapping_sub(dst as usize);
+    unsafe { ptr::copy_nonoverlapping(src, dst, to_copy) };
+}
+
 #[inline]
 fn legacy_empty_const_ptr() -> *const c_void {
     ptr::addr_of!(LEGACY_EMPTY_BYTE).cast()
@@ -2574,7 +2579,32 @@ pub unsafe extern "C" fn ZSTD_freeDCtx(dctx: *mut ZSTD_DCtx) -> usize {
 
 #[no_mangle]
 pub unsafe extern "C" fn ZSTD_copyDCtx(dst: *mut ZSTD_DCtx, src: *const ZSTD_DCtx) {
-    unsafe { ZSTD_rust_dctx_copy_prefix(dst, src) }
+    if dst.is_null() || src.is_null() {
+        return;
+    }
+    let view = unsafe { dctx_view(dst) };
+    unsafe {
+        copy_dctx_prefix(dst.cast(), src.cast(), view.in_buff);
+    }
+}
+
+#[cfg(test)]
+mod dctx_copy_tests {
+    use super::*;
+
+    #[test]
+    fn copy_prefix_stops_before_the_input_buffer_field() {
+        let source = [1u8, 2, 3, 4, 5];
+        let mut destination = [0xAAu8; 8];
+        let in_buff = unsafe { destination.as_mut_ptr().add(5).cast::<c_void>() };
+
+        unsafe {
+            copy_dctx_prefix(destination.as_mut_ptr(), source.as_ptr(), in_buff);
+        }
+
+        assert_eq!(&destination[..5], &source);
+        assert_eq!(&destination[5..], &[0xAA; 3]);
+    }
 }
 
 unsafe fn select_frame_ddict(view: &ZSTD_rustDctxView) {