Auto: s7-d2 — UDT / STRUCT / nested-DB fan-out

Closes #300

tests/ZB.MOM.WW.OtOpcUa.Driver.S7.IntegrationTests/Docker/server.py

@@ -45,10 +45,39 @@ def seed_buffer(buf: bytearray, seeds: list[dict]) -> None:
     """Poke seed values into the area buffer at declared byte offsets.
 
     Each seed is {"offset": int, "type": str, "value": int|float|bool|str}
-    where type ∈ {u8, i8, u16, i16, u32, i32, f32, bool, ascii}. Endianness is
-    big-endian (Siemens wire format).
+    where type ∈ {u8, i8, u16, i16, u32, i32, f32, bool, ascii, udt_layout}.
+    Endianness is big-endian (Siemens wire format).
+
+    PR-S7-D2: ``udt_layout`` is a meta-seed-type that flattens an ordered list
+    of UDT members into per-member primitive seeds at member-byte offsets
+    relative to the parent's ``offset``. Shape:
+
+        {
+          "offset": 400, "type": "udt_layout",
+          "members": [
+            {"name": "Pressure", "offset": 0, "type": "f32", "value": 12.5},
+            {"name": "Status",   "offset": 4, "type": "i16", "value": 7},
+            {"name": "Enabled",  "offset": 6, "type": "bool", "value": true, "bit": 0}
+          ]
+        }
+
+    Members reuse the same primitive seed types so the simulator stays
+    one-pass — ``udt_layout`` is sugar that lets the JSON profile read like
+    the UDT layout the .NET driver fan-outs into.
     """
     for seed in seeds:
+        # PR-S7-D2: expand udt_layout meta-seeds inline before the per-type
+        # dispatch so members hit the same primitive paths as a flat seed list.
+        if seed.get("type") == "udt_layout":
+            base = int(seed["offset"])
+            members = seed.get("members", [])
+            expanded = []
+            for m in members:
+                child = dict(m)
+                child["offset"] = base + int(m.get("offset", 0))
+                expanded.append(child)
+            seed_buffer(buf, expanded)
+            continue
         off = int(seed["offset"])
         t = seed["type"]
         v = seed["value"]