feat(rust): export public sequence helpers

Move the context-free ZSTD_sequenceBound and ZSTD_mergeBlockDelimiters APIs
into the Rust compression helper module. Preserve the public sequence ABI,
block-delimiter semantics, and unsigned literal-length wrapping behavior while
leaving context-driven sequence generation in C.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo test zstd_compress_api::tests -- --nocapture
- cargo test --target i686-unknown-linux-gnu zstd_compress_api::tests

Refs: Rust compression-bound export 3e2635f4

rust/src/zstd_compress_api.rs

@@ -12,6 +12,18 @@ const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00_FF00_FF00;
 const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00;
 
 const SMALL_INPUT_THRESHOLD: usize = 128 << 10;
+const ZSTD_MINMATCH_MIN: usize = 3;
+const ZSTD_BLOCKSIZE_MAX_MIN: usize = 1 << 10;
+
+/// ABI-compatible `ZSTD_Sequence` from `zstd.h`.
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct ZstdSequence {
+    offset: u32,
+    lit_length: u32,
+    match_length: u32,
+    rep: u32,
+}
 
 /// Returns the maximum one-pass compressed size, matching
 /// `ZSTD_COMPRESSBOUND()` and its public error contract.
@@ -29,6 +41,40 @@ pub extern "C" fn ZSTD_compressBound(src_size: usize) -> usize {
     src_size.wrapping_add(src_size >> 8).wrapping_add(margin)
 }
 
+/// Returns the maximum number of public sequences generated for an input.
+#[no_mangle]
+pub extern "C" fn ZSTD_sequenceBound(src_size: usize) -> usize {
+    (src_size / ZSTD_MINMATCH_MIN + 1) + (src_size / ZSTD_BLOCKSIZE_MAX_MIN + 1)
+}
+
+/// Removes explicit block delimiters, carrying their literals into the next
+/// sequence exactly as the C public helper does.
+///
+/// # Safety
+///
+/// When `sequences_size` is nonzero, `sequences` must reference that many
+/// initialized `ZSTD_Sequence` values.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_mergeBlockDelimiters(
+    sequences: *mut ZstdSequence,
+    sequences_size: usize,
+) -> usize {
+    let mut output = 0usize;
+    for input in 0..sequences_size {
+        let sequence = unsafe { sequences.add(input).read() };
+        if sequence.offset == 0 && sequence.match_length == 0 {
+            if input != sequences_size - 1 {
+                let next = unsafe { &mut *sequences.add(input + 1) };
+                next.lit_length = next.lit_length.wrapping_add(sequence.lit_length);
+            }
+        } else {
+            unsafe { sequences.add(output).write(sequence) };
+            output += 1;
+        }
+    }
+    output
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -42,4 +88,49 @@ mod tests {
         assert_eq!(ZSTD_compressBound(SMALL_INPUT_THRESHOLD), 131_584);
         assert!(ERR_isError(ZSTD_compressBound(ZSTD_MAX_INPUT_SIZE)));
     }
+
+    #[test]
+    fn sequence_bound_matches_the_public_formula() {
+        assert_eq!(ZSTD_sequenceBound(0), 2);
+        assert_eq!(ZSTD_sequenceBound(3), 3);
+        assert_eq!(ZSTD_sequenceBound(1024), 344);
+    }
+
+    #[test]
+    fn merge_block_delimiters_carries_literals_forward() {
+        let mut sequences = [
+            ZstdSequence {
+                offset: 0,
+                lit_length: 3,
+                match_length: 0,
+                rep: 0,
+            },
+            ZstdSequence {
+                offset: 7,
+                lit_length: 1,
+                match_length: 4,
+                rep: 2,
+            },
+            ZstdSequence {
+                offset: 0,
+                lit_length: 9,
+                match_length: 0,
+                rep: 0,
+            },
+        ];
+
+        assert_eq!(
+            unsafe { ZSTD_mergeBlockDelimiters(sequences.as_mut_ptr(), sequences.len()) },
+            1
+        );
+        assert_eq!(
+            sequences[0],
+            ZstdSequence {
+                offset: 7,
+                lit_length: 4,
+                match_length: 4,
+                rep: 2,
+            }
+        );
+    }
 }