refactor(mt): export window overlap policy from Rust

Move the MT LDM overlap predicate behind the original
ZSTDMT_doesOverlapWindow() by-value ABI. Rust now owns the half-open range
calculation through a checked representation of Buffer and ZSTD_window_t;
C retains the synchronization and context orchestration around the callback.

Test Plan:
- `cc -fsyntax-only -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/compress/zstdmt_compress.c` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed.
- `git diff --check` and `git diff --cached --check` -- passed.
- Heavy verification intentionally deferred until the combined seam set is ready.

rust/src/zstdmt_compress.rs

@@ -3420,6 +3420,27 @@ pub struct ZstdMtBuffer {
     pub capacity: usize,
 }
 
+/// ABI-compatible representation of the private C `ZSTD_window_t` used by
+/// the MT LDM overlap callback.
+#[repr(C)]
+#[derive(Clone, Copy, Default)]
+pub struct ZstdMtWindow {
+    pub nextSrc: *const u8,
+    pub base: *const u8,
+    pub dictBase: *const u8,
+    pub dictLimit: u32,
+    pub lowLimit: u32,
+    pub nbOverflowCorrections: u32,
+}
+
+const _: () = {
+    assert!(std::mem::offset_of!(ZstdMtWindow, nextSrc) == 0);
+    assert!(std::mem::offset_of!(ZstdMtWindow, base) == size_of::<usize>());
+    assert!(std::mem::offset_of!(ZstdMtWindow, dictBase) == 2 * size_of::<usize>());
+    assert!(std::mem::offset_of!(ZstdMtWindow, dictLimit) == 3 * size_of::<usize>());
+    assert!(size_of::<ZstdMtWindow>() == if size_of::<usize>() == 8 { 40 } else { 24 });
+};
+
 /// ABI-compatible representation of `rawSeq` from `zstd_compress_internal.h`.
 #[repr(C)]
 #[derive(Clone, Copy, Default)]
@@ -3548,6 +3569,24 @@ pub extern "C" fn ZSTDMT_rust_doesOverlapWindow(
         ) != 0) as c_int
 }
 
+/// Return non-zero when a buffer overlaps either range in a C window.
+///
+/// This keeps the original by-value internal ABI while the scalar helper
+/// above remains available to projections that cannot expose the private
+/// window layout.
+#[no_mangle]
+pub extern "C" fn ZSTDMT_doesOverlapWindow(buffer: ZstdMtBuffer, window: ZstdMtWindow) -> c_int {
+    ZSTDMT_rust_doesOverlapWindow(
+        buffer.start as *const c_void,
+        buffer.capacity,
+        window.nextSrc.cast(),
+        window.base.cast(),
+        window.dictBase.cast(),
+        window.dictLimit,
+        window.lowLimit,
+    )
+}
+
 #[derive(Default)]
 struct BufferPoolState {
     buffer_size: usize,
@@ -7850,6 +7889,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn direct_overlap_window_abi_matches_scalar_policy() {
+        let bytes = [0u8; 32];
+        let base = bytes.as_ptr();
+        let window = ZstdMtWindow {
+            nextSrc: base.wrapping_add(16),
+            base,
+            dictBase: base.wrapping_add(16),
+            dictLimit: 8,
+            lowLimit: 4,
+            nbOverflowCorrections: 0,
+        };
+        let buffer = ZstdMtBuffer {
+            start: base.wrapping_add(20) as *mut c_void,
+            capacity: 4,
+        };
+        assert_eq!(ZSTDMT_doesOverlapWindow(buffer, window), 1);
+        assert_eq!(
+            ZSTDMT_doesOverlapWindow(
+                ZstdMtBuffer {
+                    start: base as *mut u8 as *mut c_void,
+                    capacity: 4,
+                },
+                window,
+            ),
+            0
+        );
+    }
+
     #[test]
     fn buffer_pool_reuses_and_resizes_buffers() {
         let pool = unsafe { ZSTDMT_rust_buffer_pool_create(2, DEFAULT_MEM) };