feat(compress): move resetCStream policy into Rust

Move the public CStream reset wrapper's pledge normalization, callback order,
and error short-circuiting into Rust. Keep the C-owned context reset and
pledged-size mutation behind opaque callbacks with an explicit ABI projection.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml reset_cstream -- --test-threads=1
- ulimit -v 41943040; make -B -C programs -j1 zstd
- git diff --cached --check

rust/src/zstd_compress.rs

@@ -803,6 +803,60 @@ pub unsafe extern "C" fn ZSTD_rust_compress2(
     unsafe { compress2_body_with(&*state, dst, dst_capacity, src, src_size) }
 }
 
+type ResetCStreamResetFn = unsafe extern "C" fn(*mut c_void) -> usize;
+type ResetCStreamSetPledgedSrcSizeFn = unsafe extern "C" fn(*mut c_void, u64) -> usize;
+
+/// Explicit projection for the public `ZSTD_resetCStream` wrapper.
+///
+/// Rust owns the zero-to-unknown conversion, callback ordering, and error
+/// propagation. The opaque callback context stays in C, where the callbacks
+/// retain access to the private `ZSTD_CCtx` layout.
+#[repr(C)]
+pub struct ZSTD_rust_resetCStreamState {
+    callback_context: *mut c_void,
+    reset_session: ResetCStreamResetFn,
+    set_pledged_src_size: ResetCStreamSetPledgedSrcSizeFn,
+}
+
+const _: () = {
+    assert!(size_of::<ResetCStreamResetFn>() == size_of::<usize>());
+    assert!(size_of::<ResetCStreamSetPledgedSrcSizeFn>() == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_resetCStreamState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_resetCStreamState, reset_session) == size_of::<usize>());
+    assert!(
+        offset_of!(ZSTD_rust_resetCStreamState, set_pledged_src_size) == 2 * size_of::<usize>()
+    );
+    assert!(size_of::<ZSTD_rust_resetCStreamState>() == 3 * size_of::<usize>());
+};
+
+/// Drive the public `ZSTD_resetCStream` policy through C-owned callbacks.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_resetCStream(
+    state: *const ZSTD_rust_resetCStreamState,
+    pss: u64,
+) -> usize {
+    if state.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let state = unsafe { &*state };
+    let pledged_src_size = if pss == 0 {
+        ZSTD_CONTENTSIZE_UNKNOWN
+    } else {
+        pss
+    };
+
+    let reset_result = unsafe { (state.reset_session)(state.callback_context) };
+    if ERR_isError(reset_result) {
+        return reset_result;
+    }
+    let pledged_size_result =
+        unsafe { (state.set_pledged_src_size)(state.callback_context, pledged_src_size) };
+    if ERR_isError(pledged_size_result) {
+        return pledged_size_result;
+    }
+    0
+}
+
 type CompressStreamBlockFn =
     unsafe extern "C" fn(*mut c_void, *mut c_void, usize, *const c_void, usize) -> usize;
 type CompressStreamResetFn = unsafe extern "C" fn(*mut c_void) -> usize;
@@ -8559,6 +8613,94 @@ mod tests {
         assert_eq!(dict_limit, 0x89ab_cdef);
     }
 
+    #[derive(Default)]
+    struct ResetCStreamTestContext {
+        events: Vec<&'static str>,
+        reset_result: usize,
+        pledged_result: usize,
+        pledged_src_size: u64,
+    }
+
+    unsafe extern "C" fn reset_cstream_test_reset(context: *mut c_void) -> usize {
+        let context = unsafe { &mut *context.cast::<ResetCStreamTestContext>() };
+        context.events.push("reset");
+        context.reset_result
+    }
+
+    unsafe extern "C" fn reset_cstream_test_set_pledged_src_size(
+        context: *mut c_void,
+        pledged_src_size: u64,
+    ) -> usize {
+        let context = unsafe { &mut *context.cast::<ResetCStreamTestContext>() };
+        context.events.push("pledged");
+        context.pledged_src_size = pledged_src_size;
+        context.pledged_result
+    }
+
+    fn reset_cstream_test_state(
+        context: &mut ResetCStreamTestContext,
+    ) -> ZSTD_rust_resetCStreamState {
+        ZSTD_rust_resetCStreamState {
+            callback_context: (context as *mut ResetCStreamTestContext).cast(),
+            reset_session: reset_cstream_test_reset,
+            set_pledged_src_size: reset_cstream_test_set_pledged_src_size,
+        }
+    }
+
+    #[test]
+    fn reset_cstream_converts_zero_pledge_to_unknown_after_reset() {
+        let mut context = ResetCStreamTestContext::default();
+        let state = reset_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_resetCStream(&state, 0) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["reset", "pledged"]);
+        assert_eq!(context.pledged_src_size, ZSTD_CONTENTSIZE_UNKNOWN);
+    }
+
+    #[test]
+    fn reset_cstream_forwards_nonzero_pledge() {
+        let mut context = ResetCStreamTestContext::default();
+        let state = reset_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_resetCStream(&state, 123) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["reset", "pledged"]);
+        assert_eq!(context.pledged_src_size, 123);
+    }
+
+    #[test]
+    fn reset_cstream_stops_before_pledge_after_reset_error() {
+        let mut context = ResetCStreamTestContext {
+            reset_result: ERROR(ZstdErrorCode::MemoryAllocation),
+            ..ResetCStreamTestContext::default()
+        };
+        let state = reset_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_resetCStream(&state, 123) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::MemoryAllocation));
+        assert_eq!(context.events, ["reset"]);
+        assert_eq!(context.pledged_src_size, 0);
+    }
+
+    #[test]
+    fn reset_cstream_propagates_pledge_error_after_reset() {
+        let mut context = ResetCStreamTestContext {
+            pledged_result: ERROR(ZstdErrorCode::StageWrong),
+            ..ResetCStreamTestContext::default()
+        };
+        let state = reset_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_resetCStream(&state, 123) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::StageWrong));
+        assert_eq!(context.events, ["reset", "pledged"]);
+        assert_eq!(context.pledged_src_size, 123);
+    }
+
     #[test]
     fn pledged_src_size_writes_the_init_stage_value_plus_one() {
         let mut pledged_src_size_plus_one = 0;