feat(compress): route CDict end through Rust

Share the existing end-state projection between the public C wrapper and
Rust-owned compressed-dictionary orchestration. The CDict callback now reaches
ZSTD_rust_compressEnd through the private dispatcher without re-entering the
public C end wrapper, while the exported API keeps its compatibility wrapper.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo fmt --manifest-path rust/Cargo.toml --all -- --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --lib
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test-zstream ZSTREAM_TESTTIME=-T2s
- ulimit -v 41943040; make -j1 -C tests test-fuzzer FUZZERTEST=-T3s FUZZER_FLAGS=--no-big-tests

lib/compress/zstd_compress.c

@@ -6205,9 +6205,9 @@ static void ZSTD_rust_compressEnd_trace(void* context, size_t extraCSize)
     ZSTD_CCtx_trace((ZSTD_CCtx*)context, extraCSize);
 }
 
-size_t ZSTD_compressEnd_public(ZSTD_CCtx* cctx,
-                               void* dst, size_t dstCapacity,
-                         const void* src, size_t srcSize)
+static size_t ZSTD_compressEnd_dispatch(ZSTD_CCtx* cctx,
+                                         void* dst, size_t dstCapacity,
+                                   const void* src, size_t srcSize)
 {
     ZSTD_rust_compressContinueContext continueContext;
     ZSTD_rust_compressEndState state;
@@ -6229,6 +6229,13 @@ size_t ZSTD_compressEnd_public(ZSTD_CCtx* cctx,
             &state, dst, dstCapacity, src, srcSize);
 }
 
+size_t ZSTD_compressEnd_public(ZSTD_CCtx* cctx,
+                               void* dst, size_t dstCapacity,
+                         const void* src, size_t srcSize)
+{
+    return ZSTD_compressEnd_dispatch(cctx, dst, dstCapacity, src, srcSize);
+}
+
 /* NOTE: Must just wrap ZSTD_compressEnd_public() */
 size_t ZSTD_compressEnd(ZSTD_CCtx* cctx,
                         void* dst, size_t dstCapacity,
@@ -6939,7 +6946,7 @@ static size_t ZSTD_rust_compressUsingCDict_end(
         void* context, void* dst, size_t dstCapacity,
         const void* src, size_t srcSize)
 {
-    return ZSTD_compressEnd_public(
+    return ZSTD_compressEnd_dispatch(
             (ZSTD_CCtx*)context, dst, dstCapacity, src, srcSize);
 }