feat(compress): move CDict free policy into Rust

Move public CDict null handling, workspace/object teardown ordering, and
embedded-object selection behind a Rust-owned boundary. Keep private workspace
and allocator teardown in C callbacks.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release free_cdict -- --nocapture
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --release --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_compress_dictionary.rs

@@ -437,6 +437,48 @@ pub unsafe extern "C" fn ZSTD_rust_createCDict(
     cdict
 }
 
+type FreeCDictWorkspaceFn = unsafe extern "C" fn(*mut c_void);
+type FreeCDictObjectFn = unsafe extern "C" fn(*mut c_void);
+
+/// Explicit projection for the public `ZSTD_freeCDict` wrapper.
+///
+/// Rust owns null handling, workspace/object ordering, and the embedded-object
+/// decision. C retains private workspace and allocator teardown callbacks.
+#[repr(C)]
+pub struct ZSTD_rust_freeCDictState {
+    callback_context: *mut c_void,
+    cdict_in_workspace: c_int,
+    free_workspace: FreeCDictWorkspaceFn,
+    free_object: FreeCDictObjectFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_freeCDictState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_freeCDictState, cdict_in_workspace) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCDictState, free_workspace) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_freeCDictState, free_object) == 3 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_freeCDictState>() == size_of::<[usize; 4]>());
+};
+
+/// Free private CDict storage and, when applicable, its outer object.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_freeCDict(state: *const ZSTD_rust_freeCDictState) -> usize {
+    if state.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let state = unsafe { &*state };
+    if state.callback_context.is_null() {
+        return 0;
+    }
+    unsafe {
+        (state.free_workspace)(state.callback_context);
+        if state.cdict_in_workspace == 0 {
+            (state.free_object)(state.callback_context);
+        }
+    }
+    0
+}
+
 type CompressBeginUsingDictBeginFn =
     unsafe extern "C" fn(*mut c_void, *const c_void, usize, *const c_void, u64) -> usize;
 
@@ -2834,6 +2876,69 @@ mod tests {
         assert!(probe.published_cdict.is_null());
     }
 
+    #[derive(Default)]
+    struct FreeCDictProbe {
+        events: Vec<&'static str>,
+    }
+
+    unsafe extern "C" fn free_cdict_test_workspace(context: *mut c_void) {
+        let probe = unsafe { &mut *context.cast::<FreeCDictProbe>() };
+        probe.events.push("workspace");
+    }
+
+    unsafe extern "C" fn free_cdict_test_object(context: *mut c_void) {
+        let probe = unsafe { &mut *context.cast::<FreeCDictProbe>() };
+        probe.events.push("object");
+    }
+
+    fn free_cdict_test_state(
+        probe: &mut FreeCDictProbe,
+        cdict_in_workspace: c_int,
+    ) -> ZSTD_rust_freeCDictState {
+        ZSTD_rust_freeCDictState {
+            callback_context: (probe as *mut FreeCDictProbe).cast(),
+            cdict_in_workspace,
+            free_workspace: free_cdict_test_workspace,
+            free_object: free_cdict_test_object,
+        }
+    }
+
+    #[test]
+    fn free_cdict_frees_workspace_before_an_external_object() {
+        let mut probe = FreeCDictProbe::default();
+        let state = free_cdict_test_state(&mut probe, 0);
+
+        let result = unsafe { ZSTD_rust_freeCDict(&state) };
+
+        assert_eq!(result, 0);
+        assert_eq!(probe.events, ["workspace", "object"]);
+    }
+
+    #[test]
+    fn free_cdict_skips_the_external_object_when_embedded_in_workspace() {
+        let mut probe = FreeCDictProbe::default();
+        let state = free_cdict_test_state(&mut probe, 1);
+
+        let result = unsafe { ZSTD_rust_freeCDict(&state) };
+
+        assert_eq!(result, 0);
+        assert_eq!(probe.events, ["workspace"]);
+    }
+
+    #[test]
+    fn free_cdict_accepts_a_null_context() {
+        let state = ZSTD_rust_freeCDictState {
+            callback_context: ptr::null_mut(),
+            cdict_in_workspace: 0,
+            free_workspace: free_cdict_test_workspace,
+            free_object: free_cdict_test_object,
+        };
+
+        let result = unsafe { ZSTD_rust_freeCDict(&state) };
+
+        assert_eq!(result, 0);
+    }
+
     #[derive(Default)]
     struct CompressBeginUsingDictProbe {
         events: Vec<&'static str>,