feat(compress): move sequence-store validation to Rust

Keep the C DEBUGLEVEL gate and delegate the read-only match-length validation through the existing SeqStore ABI. Preserve long-match side-band decoding and the original 3-or-4 byte lower-bound assertion.

Test Plan:

- cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression

- cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression --all-targets

- cargo +nightly fmt --manifest-path rust/Cargo.toml

- make -B -C lib -j2 lib

- make -C tests -j2 fuzzer

- tests/fuzzer -s4560 -t47 -i48 -v

lib/compress/zstd_compress.c

@@ -277,6 +277,7 @@ void ZSTD_rust_storeLastLiterals(SeqStore_t* seqStorePtr,
 void ZSTD_rust_resetSeqStore(SeqStore_t* ssPtr);
 size_t ZSTD_rust_fastSequenceLengthSum(const ZSTD_Sequence* seqBuf,
                                        size_t seqBufSize);
+void ZSTD_rust_validateSeqStore(const SeqStore_t* seqStore, U32 minMatch);
 size_t ZSTD_rust_convertSequencesNoRepcodes(
         SeqDef* dstSeqs, const ZSTD_Sequence* inSeqs, size_t nbSequences);
 BlockSummary ZSTD_rust_get1BlockSummary(const ZSTD_Sequence* seqs,
@@ -2348,15 +2349,7 @@ static size_t ZSTD_fastSequenceLengthSum(ZSTD_Sequence const* seqBuf, size_t seq
 static void ZSTD_validateSeqStore(const SeqStore_t* seqStore, const ZSTD_compressionParameters* cParams)
 {
 #if DEBUGLEVEL >= 1
-    const SeqDef* seq = seqStore->sequencesStart;
-    const SeqDef* const seqEnd = seqStore->sequences;
-    size_t const matchLenLowerBound = cParams->minMatch == 3 ? 3 : 4;
-    for (; seq < seqEnd; ++seq) {
-        const ZSTD_SequenceLength seqLength = ZSTD_getSequenceLength(seqStore, seq);
-        assert(seqLength.matchLength >= matchLenLowerBound);
-        (void)seqLength;
-        (void)matchLenLowerBound;
-    }
+    ZSTD_rust_validateSeqStore(seqStore, (U32)cParams->minMatch);
 #else
     (void)seqStore;
     (void)cParams;

rust/src/zstd_compress_stats.rs

@@ -1151,6 +1151,31 @@ pub unsafe extern "C" fn ZSTD_rust_fastSequenceLengthSum(
     lit_len_sum.wrapping_add(match_len_sum)
 }
 
+/// Validates the match lengths produced in a sequence store.  This is the
+/// Rust leaf for C's `ZSTD_validateSeqStore()`; the C wrapper retains the
+/// `DEBUGLEVEL` gate so release builds keep the original no-op behavior.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_validateSeqStore(
+    seq_store: *const SeqStore_t,
+    min_match: c_uint,
+) {
+    let seq_store = unsafe { &*seq_store };
+    let match_len_lower_bound = if min_match == 3 { 3 } else { 4 };
+    let nb_sequences =
+        unsafe { seq_store.sequences.offset_from(seq_store.sequencesStart) } as usize;
+
+    for index in 0..nb_sequences {
+        let sequence = unsafe { *seq_store.sequencesStart.add(index) };
+        let mut match_length = sequence.mlBase as usize + MINMATCH;
+        if index == seq_store.longLengthPos as usize
+            && seq_store.longLengthType == ZSTD_LLT_MATCH_LENGTH
+        {
+            match_length += 0x10000;
+        }
+        assert!(match_length >= match_len_lower_bound);
+    }
+}
+
 /// Returns whether all bytes in the input have the same value.
 ///
 /// This keeps the original C helper's precondition that `src` points to at
@@ -3378,6 +3403,45 @@ mod tests {
         );
     }
 
+    #[test]
+    fn validate_seq_store_accepts_ordinary_match_lengths() {
+        let mut fixture = TestSeqStore::new(
+            vec![
+                SeqDef {
+                    offBase: 1,
+                    litLength: 0,
+                    mlBase: 0,
+                },
+                SeqDef {
+                    offBase: 1,
+                    litLength: 0,
+                    mlBase: 1,
+                },
+            ],
+            vec![],
+        );
+        let seq_store = fixture.seq_store();
+
+        unsafe { ZSTD_rust_validateSeqStore(&seq_store, 3) };
+    }
+
+    #[test]
+    fn validate_seq_store_accounts_for_long_match_metadata() {
+        let mut fixture = TestSeqStore::new(
+            vec![SeqDef {
+                offBase: 1,
+                litLength: 0,
+                mlBase: 0,
+            }],
+            vec![],
+        );
+        let mut seq_store = fixture.seq_store();
+        seq_store.longLengthType = ZSTD_LLT_MATCH_LENGTH;
+        seq_store.longLengthPos = 0;
+
+        unsafe { ZSTD_rust_validateSeqStore(&seq_store, 4) };
+    }
+
     #[test]
     fn post_process_rejects_external_count_overflow() {
         let mut out = [ZSTD_Sequence {