feat(compress): move external sequence store reset into Rust

Move the mutable external raw-sequence-store assignment and cursor reset
behind a Rust-owned pointer projection. Keep the public C entry point and
its stage/LDM assertions in C, without exposing RawSeqStore_t to Rust.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused reference_external_sequences unit tests

lib/compress/zstd_compress.c

@@ -1390,6 +1390,30 @@ typedef char ZSTD_rust_external_sequence_producer_state_layout[
                 == 13 * sizeof(void*))
         ? 1 : -1];
 
+typedef struct {
+    rawSeq** seq;
+    size_t* pos;
+    size_t* posInSequence;
+    size_t* size;
+    size_t* capacity;
+} ZSTD_rust_externalSequenceStoreState;
+void ZSTD_rust_referenceExternalSequences(
+        const ZSTD_rust_externalSequenceStoreState* state,
+        rawSeq* seq, size_t nbSeq);
+typedef char ZSTD_rust_external_sequence_store_state_layout[
+        (offsetof(ZSTD_rust_externalSequenceStoreState, seq) == 0
+         && offsetof(ZSTD_rust_externalSequenceStoreState, pos)
+                == sizeof(void*)
+         && offsetof(ZSTD_rust_externalSequenceStoreState, posInSequence)
+                == 2 * sizeof(void*)
+         && offsetof(ZSTD_rust_externalSequenceStoreState, size)
+                == 3 * sizeof(void*)
+         && offsetof(ZSTD_rust_externalSequenceStoreState, capacity)
+                == 4 * sizeof(void*)
+         && sizeof(ZSTD_rust_externalSequenceStoreState)
+                == 5 * sizeof(void*))
+        ? 1 : -1];
+
 /* The sequence-compression loop receives only the state it actually reads or
  * updates.  In particular, neither ZSTD_CCtx nor a C function pointer crosses
  * the Rust ABI. */
@@ -4066,11 +4090,15 @@ void ZSTD_referenceExternalSequences(ZSTD_CCtx* cctx, rawSeq* seq, size_t nbSeq)
 {
     assert(cctx->stage == ZSTDcs_init);
     assert(nbSeq == 0 || cctx->appliedParams.ldmParams.enableLdm != ZSTD_ps_enable);
-    cctx->externSeqStore.seq = seq;
-    cctx->externSeqStore.size = nbSeq;
-    cctx->externSeqStore.capacity = nbSeq;
-    cctx->externSeqStore.pos = 0;
-    cctx->externSeqStore.posInSequence = 0;
+    {
+        ZSTD_rust_externalSequenceStoreState state;
+        state.seq = &cctx->externSeqStore.seq;
+        state.pos = &cctx->externSeqStore.pos;
+        state.posInSequence = &cctx->externSeqStore.posInSequence;
+        state.size = &cctx->externSeqStore.size;
+        state.capacity = &cctx->externSeqStore.capacity;
+        ZSTD_rust_referenceExternalSequences(&state, seq, nbSeq);
+    }
 }