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.

lib/compress/zstdmt_compress.c

@@ -2910,21 +2910,7 @@ static Range ZSTDMT_getInputDataInUse(ZSTDMT_CCtx* mtctx)
     return (Range){ range.start, range.size };
 }
 
-static int ZSTDMT_doesOverlapWindow(Buffer buffer, ZSTD_window_t window)
-{
-    DEBUGLOG(5, "ZSTDMT_doesOverlapWindow");
-    DEBUGLOG(5, "extDict [0x%zx, 0x%zx)",
-                (size_t)window.dictBase + window.lowLimit,
-                (size_t)window.dictBase + window.dictLimit);
-    DEBUGLOG(5, "prefix  [0x%zx, 0x%zx)",
-                (size_t)window.base + window.dictLimit,
-                (size_t)window.nextSrc);
-
-    return ZSTDMT_rust_doesOverlapWindow(buffer.start, buffer.capacity,
-                                         window.nextSrc, window.base,
-                                         window.dictBase, window.dictLimit,
-                                         window.lowLimit);
-}
+int ZSTDMT_doesOverlapWindow(Buffer buffer, ZSTD_window_t window);
 
 static void ZSTDMT_waitForLdmLock(void* opaque)
 {