WIP: checkpoint geocaching before main integration

modules/core_sys/include/sys/crc32.h

@@ -0,0 +1,23 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+
+namespace sys
+{
+// CRC-32/ISO-HDLC. Pass a previous returned CRC to continue over another span.
+// Extracted from the existing chat storage codec; default preserves its format.
+inline uint32_t crc32(const void* data, std::size_t len, uint32_t previous = 0)
+{
+    const auto* bytes = static_cast<const uint8_t*>(data);
+    uint32_t crc = ~previous;
+    for (std::size_t index = 0; index < len; ++index)
+    {
+        crc ^= bytes[index];
+        for (uint8_t bit = 0; bit < 8; ++bit)
+        {
+            crc = (crc & 1U) != 0U ? (crc >> 1U) ^ 0xEDB88320U : crc >> 1U;
+        }
+    }
+    return ~crc;
+}
+} // namespace sys