Make append buffer contiguous and update iterators

include/vase/iterators/petal.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "../shard.h"
+#include "iter.h"
+#include "pch.h"
+
+struct PetalIterator {
+  Direction dir;
+  Shard *root = nullptr;
+  std::vector<Shard *> stack;
+  Petal *petal = nullptr;
+  uint64_t global_offset = 0;
+  uint64_t last_offset = 0;
+  uint64_t petal_offset = 0;
+  uint64_t global_line = 0;
+
+  PetalIterator(Shard *r, Direction dir);
+  ~PetalIterator() = default;
+
+  void seek_offset(uint64_t offset);
+  void seek_line(uint64_t offset);
+  bool next(const char **data, uint64_t *out_len);
+  uint64_t byte_offset();
+
+private:
+  Petal *_next(uint64_t *offset);
+};