feat(compress): move CCtx custom memory copy into Rust

Replace the remaining CCtx-copy custom-memory callback with explicit source and
destination pointers. Rust now copies the ABI-checked ZSTD_customMem payload in
its original position before reset, while C retains private context access and
the surrounding workspace and table operations.

Test Plan:
- ulimit -v 41943040 cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040 CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml
- ulimit -v 41943040 make -j1
- ulimit -v 41943040 make -j1 -C tests test-zstream ZSTREAM_TESTTIME=-T2s
- ulimit -v 41943040 make -j1 -C tests test-fuzzer FUZZERTEST=-T3s FUZZER_FLAGS=--no-big-tests

lib/compress/zstd_compress.c

@@ -459,8 +459,6 @@ typedef char ZSTD_rust_reset_cctx_state_layout[
         ? 1 : -1];
 typedef size_t (*ZSTD_rust_copyCCtxCheckStage_f)(
         void* context, const void* srcCCtx);
-typedef void (*ZSTD_rust_copyCCtxCopyState_f)(
-        void* context, const void* srcCCtx);
 typedef size_t (*ZSTD_rust_copyCCtxReset_f)(
         void* context, const void* srcCCtx,
         const ZSTD_frameParameters* fParams,
@@ -474,7 +472,8 @@ typedef struct {
     const ZSTD_frameParameters* fParams;
     U64 pledgedSrcSize;
     ZSTD_rust_copyCCtxCheckStage_f checkStage;
-    ZSTD_rust_copyCCtxCopyState_f copyCustomMem;
+    void* destinationCustomMem;
+    const void* sourceCustomMem;
     ZSTD_rust_copyCCtxReset_f reset;
     ZSTD_rust_copyCCtxMarkTables_f markTablesDirty;
     ZSTD_rust_copyCCtxCopyTables_f copyTables;
@@ -536,7 +535,7 @@ typedef char ZSTD_rust_copy_cctx_internal_state_layout[
                 == 3 * sizeof(void*) + sizeof(U64)
          && offsetof(ZSTD_rust_copyCCtxInternalState, zbuff)
                 == 3 * sizeof(void*) + sizeof(U64)
-                 + 18 * sizeof(void*)
+                 + 19 * sizeof(void*)
          && sizeof(ZSTD_rust_copyCCtxInternalState)
                 == ((offsetof(ZSTD_rust_copyCCtxInternalState, zbuff)
                      + sizeof(int) + sizeof(void*) - 1) / sizeof(void*))
@@ -4756,14 +4755,6 @@ static size_t ZSTD_rust_copyCCtx_check_stage(
     return 0;
 }
 
-static void ZSTD_rust_copyCCtx_copy_custom_mem(
-        void* context, const void* srcCCtx)
-{
-    ZSTD_CCtx* const dst = (ZSTD_CCtx*)context;
-    const ZSTD_CCtx* const src = (const ZSTD_CCtx*)srcCCtx;
-    ZSTD_memcpy(&dst->customMem, &src->customMem, sizeof(ZSTD_customMem));
-}
-
 static size_t ZSTD_rust_copyCCtx_reset(
         void* context, const void* srcCCtx,
         const ZSTD_frameParameters* fParams,
@@ -4846,7 +4837,10 @@ static size_t ZSTD_rust_copyCCtx_internal_callback(
     state.fParams = fParams;
     state.pledgedSrcSize = pledgedSrcSize;
     state.checkStage = ZSTD_rust_copyCCtx_check_stage;
-    state.copyCustomMem = ZSTD_rust_copyCCtx_copy_custom_mem;
+    state.destinationCustomMem =
+            &((ZSTD_CCtx*)context)->customMem;
+    state.sourceCustomMem =
+            &((const ZSTD_CCtx*)srcCCtx)->customMem;
     state.reset = ZSTD_rust_copyCCtx_reset;
     state.markTablesDirty = ZSTD_rust_copyCCtx_mark_tables_dirty;
     state.copyTables = ZSTD_rust_copyCCtx_copy_tables;