feat(decompress): move sequence decoder policy into Rust

Port sequence-decoder selection and offset-history policy into Rust. The
Rust block decoder now handles short/long selection, offset-table analysis,
history thresholds, and prefetch sequencing; C projects only the decoder mode
and private context fields required by the ABI.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --all-targets -- --test-threads=1
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C lib -j2 lib ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT=1 ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG=0
- make -B -C lib -j2 lib ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT=0 ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG=1
- make -B -C tests -j2 test-zstd

lib/decompress/zstd_decompress_block.c

@@ -20,6 +20,19 @@
 #include "zstd_decompress_internal.h"
 #include "zstd_decompress_block.h"
 
+#if defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT) && \
+    defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
+#error "Cannot force the use of the short and the long ZSTD_decompressSequences variants!"
+#endif
+
+#if defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT)
+#define ZSTD_RUST_SEQUENCE_DECODER_MODE 1
+#elif defined(ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG)
+#define ZSTD_RUST_SEQUENCE_DECODER_MODE 2
+#else
+#define ZSTD_RUST_SEQUENCE_DECODER_MODE 0
+#endif
+
 typedef char ZSTD_rust_block_seq_symbol_layout[(sizeof(ZSTD_seqSymbol) == 8) ? 1 : -1];
 typedef char ZSTD_rust_block_entropy_rep_offset[
         (offsetof(ZSTD_entropyDTables_t, rep) == 26652) ? 1 : -1];
@@ -53,6 +66,7 @@ typedef struct {
     ZSTD_litLocation_e* litBufferLocation;
     BYTE* litExtraBuffer;
     size_t litExtraBufferSize;
+    int sequenceDecoderMode;
 } ZSTD_rustBlockCtx;
 
 static ZSTD_rustBlockCtx ZSTD_rust_block_context(ZSTD_DCtx* dctx)
@@ -84,6 +98,7 @@ static ZSTD_rustBlockCtx ZSTD_rust_block_context(ZSTD_DCtx* dctx)
     ctx.litBufferLocation = &dctx->litBufferLocation;
     ctx.litExtraBuffer = dctx->litExtraBuffer;
     ctx.litExtraBufferSize = ZSTD_LITBUFFEREXTRASIZE;
+    ctx.sequenceDecoderMode = ZSTD_RUST_SEQUENCE_DECODER_MODE;
     return ctx;
 }