feat(compress): move end-stream estimate to Rust

Keep ZSTD_endStream's streaming call, error propagation, multithreaded
minimal estimate, logging, and context state in C. Move only the final
single-thread estimate behind a scalar Rust ABI helper so the frame-ended
branch and raw checksum flag are explicit while size_t arithmetic wraps as
in C.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed before and after formatting.
- The same clippy command with `--benches` and `--tests` -- passed before and after formatting.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed.
- Focused end-stream Rust tests -- 4 passed.
- `make -B -C lib -j2 lib` -- passed.
- `make -C tests test-rust-lib-smoke` -- passed.
- `tests/fuzzer -s4560 -t56 -i57 -v` -- passed.
- `make -C tests -j2 test-zstream` -- passed: 84 named, 6,457, and 8,692 randomized tests. The pre-existing unterminated-string warning remains.
- `git diff --check` and `git diff --cached --check` -- passed.

lib/compress/zstd_compress.c

@@ -106,6 +106,8 @@ ZSTD_inBuffer ZSTD_rust_inBufferForEndFlush(int inBufferMode,
                                             const void* expectedSrc,
                                             size_t expectedSize,
                                             size_t expectedPos);
+size_t ZSTD_rust_endStreamRemaining(size_t remainingToFlush,
+                                    int frameEnded, int checksumFlag);
 size_t ZSTD_rust_checkBufferStability(
         int inBufferMode, int outBufferMode,
         const void* expectedInSrc, size_t expectedInPos,
@@ -5655,9 +5657,9 @@ size_t ZSTD_endStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output)
     FORWARD_IF_ERROR(remainingToFlush , "ZSTD_compressStream2(,,ZSTD_e_end) failed");
     if (zcs->appliedParams.nbWorkers > 0) return remainingToFlush;   /* minimal estimation */
     /* single thread mode : attempt to calculate remaining to flush more precisely */
-    {   size_t const lastBlockSize = zcs->frameEnded ? 0 : ZSTD_BLOCKHEADERSIZE;
-        size_t const checksumSize = (size_t)(zcs->frameEnded ? 0 : zcs->appliedParams.fParams.checksumFlag * 4);
-        size_t const toFlush = remainingToFlush + lastBlockSize + checksumSize;
+    {   size_t const toFlush = ZSTD_rust_endStreamRemaining(
+                remainingToFlush, zcs->frameEnded,
+                zcs->appliedParams.fParams.checksumFlag);
         DEBUGLOG(4, "ZSTD_endStream : remaining to flush : %u", (unsigned)toFlush);
         return toFlush;
     }