add header doc comments to non-obvious util data structures

util/span_writer.h

@@ -4,6 +4,9 @@
 #include <cstring>
 #include <span>
 
+// Bounded writer over a caller-owned byte buffer. Tracks fill level and a
+// sticky overflow flag so packing code can write blindly and check once at the
+// end. Unlike std::span, it has a size cursor that grows toward capacity.
 class span_writer {
     uint8_t *m_data;
     size_t m_capacity;