docs: reorder & testing

pkg/schema/fbs/common.fbs

@@ -0,0 +1,14 @@
+// common contains FlatBuffers types shared across multiple schemas
+// (order, report, …). Files that need these types include this one
+// via `include "common.fbs";` and reference them through the `common.`
+// namespace.
+namespace common;
+
+// UUID is a 128-bit RFC 4122 identifier encoded as two big-endian
+// uint64 halves (`hi` carries bytes 0..7, `lo` carries bytes 8..15).
+// Transcoders use the helpers in `pkg/transcoder/uuid.go` to convert
+// between this layout and `github.com/google/uuid.UUID`.
+struct UUID {
+  hi:uint64;
+  lo:uint64;
+}