feat(compress): move sequence leaves to Rust

Move sequence-store byte accounting, chunk derivation, and repcode resolution into the Rust compression module. Move MT raw-sequence buffer conversions into Rust while preserving the existing C adapters and ABI layouts.

Test Plan:\n- cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression\n- cargo clippy --manifest-path rust/Cargo.toml\n- cargo clippy --manifest-path rust/Cargo.toml --benches\n- cargo clippy --manifest-path rust/Cargo.toml --tests\n- make -B -C lib -j2 lib\n- make -B -C tests -j2 test-cli-tests

lib/compress/zstdmt_compress.c

@@ -115,6 +115,25 @@ void ZSTDMT_rust_buffer_pool_release(ZSTDMT_RustBufferPool* pool,
 ZSTDMT_RustBuffer ZSTDMT_rust_buffer_pool_resize(ZSTDMT_RustBufferPool* pool,
                                                   ZSTDMT_RustBuffer buffer);
 
+typedef struct {
+    rawSeq* seq;
+    size_t pos;
+    size_t posInSequence;
+    size_t size;
+    size_t capacity;
+} ZSTDMT_RustRawSeqStore;
+
+typedef char ZSTDMT_rust_raw_seq_layout[
+        (sizeof(rawSeq) == 3 * sizeof(U32)) ? 1 : -1];
+typedef char ZSTDMT_rust_raw_seq_store_layout[
+        (sizeof(ZSTDMT_RustRawSeqStore) == sizeof(RawSeqStore_t)
+         && offsetof(ZSTDMT_RustRawSeqStore, pos) == offsetof(RawSeqStore_t, pos)
+         && offsetof(ZSTDMT_RustRawSeqStore, capacity) == offsetof(RawSeqStore_t, capacity))
+                ? 1 : -1];
+
+ZSTDMT_RustRawSeqStore ZSTDMT_rust_bufferToSeq(ZSTDMT_RustBuffer buffer);
+ZSTDMT_RustBuffer ZSTDMT_rust_seqToBuffer(ZSTDMT_RustRawSeqStore seq);
+
 unsigned ZSTDMT_rust_computeTargetJobLog(unsigned windowLog, unsigned chainLog,
                                          int strategy, int enableLdm);
 int ZSTDMT_rust_overlapLog(int overlapLog, int strategy);
@@ -234,18 +253,29 @@ static size_t ZSTDMT_sizeof_seqPool(ZSTDMT_seqPool* seqPool)
 
 static RawSeqStore_t bufferToSeq(Buffer buffer)
 {
-    RawSeqStore_t seq = kNullRawSeqStore;
-    seq.seq = (rawSeq*)buffer.start;
-    seq.capacity = buffer.capacity / sizeof(rawSeq);
+    ZSTDMT_RustRawSeqStore const rustSeq =
+        ZSTDMT_rust_bufferToSeq((ZSTDMT_RustBuffer){ buffer.start, buffer.capacity });
+    RawSeqStore_t seq = {
+        (rawSeq*)rustSeq.seq,
+        rustSeq.pos,
+        rustSeq.posInSequence,
+        rustSeq.size,
+        rustSeq.capacity
+    };
     return seq;
 }
 
 static Buffer seqToBuffer(RawSeqStore_t seq)
 {
-    Buffer buffer;
-    buffer.start = seq.seq;
-    buffer.capacity = seq.capacity * sizeof(rawSeq);
-    return buffer;
+    ZSTDMT_RustRawSeqStore const rustSeq = {
+        seq.seq,
+        seq.pos,
+        seq.posInSequence,
+        seq.size,
+        seq.capacity
+    };
+    ZSTDMT_RustBuffer const rustBuffer = ZSTDMT_rust_seqToBuffer(rustSeq);
+    return (Buffer){ rustBuffer.start, rustBuffer.capacity };
 }
 
 static RawSeqStore_t ZSTDMT_getSeq(ZSTDMT_seqPool* seqPool)