feat(compress): move CCtx free policy into Rust

Move ZSTD_freeCCtx null handling, static-context rejection, content/object
free ordering, and embedded-workspace policy behind a Rust-owned ABI bridge.
C retains dictionary, multithread, workspace, and custom allocator teardown
through callbacks, and ZSTD_freeCStream continues to use the CCtx path.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused free_cctx unit tests

rust/src/zstd_compress.rs

@@ -1049,6 +1049,55 @@ pub unsafe extern "C" fn ZSTD_rust_refThreadPool(
     0
 }
 
+type FreeCCtxContentFn = unsafe extern "C" fn(*mut c_void);
+type FreeCCtxObjectFn = unsafe extern "C" fn(*mut c_void);
+
+/// Explicit projection for the public `ZSTD_freeCCtx` wrapper.
+///
+/// Rust owns the null/static/embedded-object policy and callback ordering.
+/// C retains the private content teardown and custom allocator callbacks.
+#[repr(C)]
+pub struct ZSTD_rust_freeCCtxState {
+    callback_context: *mut c_void,
+    static_size: usize,
+    cctx_in_workspace: c_int,
+    free_content: FreeCCtxContentFn,
+    free_object: FreeCCtxObjectFn,
+}
+
+const _: () = {
+    assert!(size_of::<FreeCCtxContentFn>() == size_of::<usize>());
+    assert!(size_of::<FreeCCtxObjectFn>() == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCCtxState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_freeCCtxState, static_size) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCCtxState, cctx_in_workspace) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCCtxState, free_content) == 3 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCCtxState, free_object) == 4 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_freeCCtxState>() == 5 * size_of::<usize>());
+};
+
+/// Free the C-owned context content and, when applicable, its outer object.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_freeCCtx(state: *const ZSTD_rust_freeCCtxState) -> usize {
+    if state.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let state = unsafe { &*state };
+    if state.callback_context.is_null() {
+        return 0;
+    }
+    if state.static_size != 0 {
+        return ERROR(ZstdErrorCode::MemoryAllocation);
+    }
+    unsafe {
+        (state.free_content)(state.callback_context);
+        if state.cctx_in_workspace == 0 {
+            (state.free_object)(state.callback_context);
+        }
+    }
+    0
+}
+
 type ResetCCtxClearAllDictsFn = unsafe extern "C" fn(*mut c_void);
 type ResetCCtxResetParamsFn = unsafe extern "C" fn(*mut c_void) -> usize;
 
@@ -10558,6 +10607,77 @@ mod tests {
         assert_eq!(result, ERROR(ZstdErrorCode::Generic));
     }
 
+    #[derive(Default)]
+    struct FreeCCtxTestContext {
+        events: Vec<&'static str>,
+    }
+
+    unsafe extern "C" fn free_cctx_test_content(context: *mut c_void) {
+        let context = unsafe { &mut *context.cast::<FreeCCtxTestContext>() };
+        context.events.push("content");
+    }
+
+    unsafe extern "C" fn free_cctx_test_object(context: *mut c_void) {
+        let context = unsafe { &mut *context.cast::<FreeCCtxTestContext>() };
+        context.events.push("object");
+    }
+
+    fn free_cctx_test_state(
+        context: *mut c_void,
+        static_size: usize,
+        cctx_in_workspace: c_int,
+    ) -> ZSTD_rust_freeCCtxState {
+        ZSTD_rust_freeCCtxState {
+            callback_context: context,
+            static_size,
+            cctx_in_workspace,
+            free_content: free_cctx_test_content,
+            free_object: free_cctx_test_object,
+        }
+    }
+
+    #[test]
+    fn free_cctx_frees_content_before_an_external_object() {
+        let mut context = FreeCCtxTestContext::default();
+        let state = free_cctx_test_state((&mut context as *mut FreeCCtxTestContext).cast(), 0, 0);
+
+        let result = unsafe { ZSTD_rust_freeCCtx(&state) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["content", "object"]);
+    }
+
+    #[test]
+    fn free_cctx_skips_the_external_object_when_embedded_in_workspace() {
+        let mut context = FreeCCtxTestContext::default();
+        let state = free_cctx_test_state((&mut context as *mut FreeCCtxTestContext).cast(), 0, 1);
+
+        let result = unsafe { ZSTD_rust_freeCCtx(&state) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["content"]);
+    }
+
+    #[test]
+    fn free_cctx_rejects_static_contexts_before_any_callback() {
+        let mut context = FreeCCtxTestContext::default();
+        let state = free_cctx_test_state((&mut context as *mut FreeCCtxTestContext).cast(), 1, 0);
+
+        let result = unsafe { ZSTD_rust_freeCCtx(&state) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::MemoryAllocation));
+        assert!(context.events.is_empty());
+    }
+
+    #[test]
+    fn free_cctx_accepts_a_null_context() {
+        let state = free_cctx_test_state(ptr::null_mut(), 0, 0);
+
+        let result = unsafe { ZSTD_rust_freeCCtx(&state) };
+
+        assert_eq!(result, 0);
+    }
+
     #[derive(Default)]
     struct SetCParamsTestContext {
         events: Vec<&'static str>,