feat(compress): move external sequence validation to Rust

Port the external-sequence window, offset, and minimum-match checks through a scalar C ABI leaf while retaining the C caller's control flow and diagnostics.

Test Plan: cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression; cargo clippy --manifest-path rust/Cargo.toml; cargo clippy --manifest-path rust/Cargo.toml --benches; cargo clippy --manifest-path rust/Cargo.toml --tests; make -B -C lib -j2 lib; make -B -C tests -j2 test-zstream (84 deterministic, 7063 and 8665 fuzzer cases).

lib/compress/zstd_compress.c

@@ -265,6 +265,9 @@ size_t ZSTD_rust_blockSizeExplicitDelimiter(const ZSTD_Sequence* inSeqs,
 size_t ZSTD_rust_determineBlockSize(int mode, size_t blockSize, size_t remaining,
                                     const ZSTD_Sequence* inSeqs, size_t inSeqsSize,
                                     U32 seqIdx);
+size_t ZSTD_rust_validateSequence(U32 offBase, U32 matchLength, U32 minMatch,
+                                  size_t posInSrc, U32 windowLog, size_t dictSize,
+                                  int useSequenceProducer);
 
 typedef char ZSTD_rust_stats_seqdef_layout[(sizeof(SeqDef) == 8) ? 1 : -1];
 typedef char ZSTD_rust_stats_seqstore_long_length_pos[
@@ -5339,18 +5342,9 @@ static size_t
 ZSTD_validateSequence(U32 offBase, U32 matchLength, U32 minMatch,
                       size_t posInSrc, U32 windowLog, size_t dictSize, int useSequenceProducer)
 {
-    U32 const windowSize = 1u << windowLog;
-    /* posInSrc represents the amount of data the decoder would decode up to this point.
-     * As long as the amount of data decoded is less than or equal to window size, offsets may be
-     * larger than the total length of output decoded in order to reference the dict, even larger than
-     * window size. After output surpasses windowSize, we're limited to windowSize offsets again.
-     */
-    size_t const offsetBound = posInSrc > windowSize ? (size_t)windowSize : posInSrc + (size_t)dictSize;
-    size_t const matchLenLowerBound = (minMatch == 3 || useSequenceProducer) ? 3 : 4;
-    RETURN_ERROR_IF(offBase > OFFSET_TO_OFFBASE(offsetBound), externalSequences_invalid, "Offset too large!");
-    /* Validate maxNbSeq is large enough for the given matchLength and minMatch */
-    RETURN_ERROR_IF(matchLength < matchLenLowerBound, externalSequences_invalid, "Matchlength too small for the minMatch");
-    return 0;
+    return ZSTD_rust_validateSequence(offBase, matchLength, minMatch,
+                                      posInSrc, windowLog, dictSize,
+                                      useSequenceProducer);
 }
 
 /* Returns an offset code, given a sequence's raw offset, the ongoing repcode array, and whether litLength == 0 */