feat(compress): move repcode invalidation to Rust

Keep ZSTD_invalidateRepCodes responsible for extracting the private previous
block state and checking that the match window has no external dictionary.
Delegate only the fixed three-entry repcode reset through a narrow pointer ABI,
with a C assertion preserving the Rust side's ZSTD_REP_NUM assumption.

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

lib/compress/zstd_compress.c

@@ -298,6 +298,8 @@ size_t ZSTD_rust_copyBlockSequences(
         SeqCollector* seqCollector, const SeqStore_t* seqStore,
         const U32 prevRepcodes[ZSTD_REP_NUM]);
 void ZSTD_rust_resetCompressedBlockState(ZSTD_compressedBlockState_t* bs);
+void ZSTD_rust_invalidateRepCodes(U32 rep[ZSTD_REP_NUM]);
+typedef char ZSTD_rust_invalidate_rep_count[(ZSTD_REP_NUM == 3) ? 1 : -1];
 void ZSTD_rust_confirmRepcodesAndEntropyTables(
         ZSTD_compressedBlockState_t** prevCBlock,
         ZSTD_compressedBlockState_t** nextCBlock);
@@ -1798,8 +1800,8 @@ static size_t ZSTD_resetCCtx_internal(ZSTD_CCtx* zc,
  * Note : only works with regular variant;
  *        do not use with extDict variant ! */
 void ZSTD_invalidateRepCodes(ZSTD_CCtx* cctx) {
-    int i;
-    for (i=0; i<ZSTD_REP_NUM; i++) cctx->blockState.prevCBlock->rep[i] = 0;
+    ZSTD_compressedBlockState_t* const prevCBlock = cctx->blockState.prevCBlock;
+    ZSTD_rust_invalidateRepCodes(prevCBlock->rep);
     assert(!ZSTD_window_hasExtDict(cctx->blockState.matchState.window));
 }