feat(compress): move block header serialization to Rust

Keep the private writeBlockHeader signature and its debug logging in C while
moving the pure block-type, size, and 24-bit little-endian encoding behind a
narrow Rust ABI. The Rust leaf preserves the RLE blockSize path for cSize == 1
and the compressed cSize path for every other value.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression block_header -- --nocapture` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` -- passed
- `./tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- Required clippy, nightly fmt, and diff checks -- passed

lib/compress/zstd_compress.c

@@ -37,6 +37,8 @@ size_t ZSTD_rust_writeFrameHeader(void* dst, size_t dstCapacity,
                                   int contentSizeFlag, int format,
                                   U32 windowLog, U64 pledgedSrcSize,
                                   U32 dictID);
+void ZSTD_rust_writeBlockHeader(void* op, size_t cSize, size_t blockSize,
+                                U32 lastBlock);
 size_t ZSTD_rust_writeEpilogue(void* dst, size_t dstCapacity, int* stage,
                                int noDictIDFlag, int checksumFlag,
                                int contentSizeFlag, int format, U32 windowLog,
@@ -2592,10 +2594,7 @@ ZSTD_blockState_confirmRepcodesAndEntropyTables(ZSTD_blockState_t* const bs)
 static void
 writeBlockHeader(void* op, size_t cSize, size_t blockSize, U32 lastBlock)
 {
-    U32 const cBlockHeader = cSize == 1 ?
-                        lastBlock + (((U32)bt_rle)<<1) + (U32)(blockSize << 3) :
-                        lastBlock + (((U32)bt_compressed)<<1) + (U32)(cSize << 3);
-    MEM_writeLE24(op, cBlockHeader);
+    ZSTD_rust_writeBlockHeader(op, cSize, blockSize, lastBlock);
     DEBUGLOG(5, "writeBlockHeader: cSize: %zu blockSize: %zu lastBlock: %u", cSize, blockSize, lastBlock);
 }
 

rust/src/zstd_compress_frame.rs

@@ -25,6 +25,7 @@ const ZSTD_F_ZSTD1: c_int = 0;
 const ZSTD_BLOCK_SIZE: usize = 128 << 10;
 const ZSTD_FAST: c_int = 1;
 const ZSTD_BTULTRA2: c_int = 9;
+const ZSTD_BT_COMPRESSED: u32 = 2;
 const SPLIT_LEVELS: [c_int; 10] = [0, 0, 1, 2, 2, 3, 3, 4, 4, 4];
 const ZSTD_BT_RLE: u32 = 1;
 const ZSTDCS_CREATED: c_int = 0;
@@ -83,6 +84,31 @@ pub(crate) unsafe fn write_raw_block(
     needed
 }
 
+/// Rust implementation of the private compressed-block header serializer.
+///
+/// The C wrapper retains the original static helper signature and keeps its
+/// debug logging at the call site.  The helper itself only needs scalar
+/// values and writes the low 24 bits in little-endian order, as `MEM_writeLE24`
+/// does.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_writeBlockHeader(
+    op: *mut c_void,
+    c_size: usize,
+    block_size: usize,
+    last_block: c_uint,
+) {
+    let block_type = if c_size == 1 {
+        ZSTD_BT_RLE
+    } else {
+        ZSTD_BT_COMPRESSED
+    };
+    let size = if c_size == 1 { block_size } else { c_size };
+    let header = last_block
+        .wrapping_add(block_type << 1)
+        .wrapping_add((size as u32).wrapping_shl(3));
+    unsafe { write_le24(op.cast(), header) };
+}
+
 /// Rust implementation of the private `ZSTD_writeFrameHeader()` leaf.
 ///
 /// `no_dict_id_flag`, `checksum_flag`, `content_size_flag`, `format`, and
@@ -604,6 +630,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn writes_rle_block_header() {
+        let mut output = [0u8; ZSTD_BLOCKHEADERSIZE];
+        unsafe {
+            ZSTD_rust_writeBlockHeader(output.as_mut_ptr().cast(), 1, 0x12345, 1);
+        }
+        assert_eq!(output, [0x2b, 0x1a, 0x09]);
+    }
+
+    #[test]
+    fn writes_compressed_block_header() {
+        let mut output = [0u8; ZSTD_BLOCKHEADERSIZE];
+        unsafe {
+            ZSTD_rust_writeBlockHeader(output.as_mut_ptr().cast(), 0x23456, 0x54321, 1);
+        }
+        assert_eq!(output, [0xb5, 0xa2, 0x11]);
+    }
+
     #[test]
     fn optimal_block_size_keeps_small_blocks_intact() {
         let result = unsafe {