Auto: twincat-4.1 — nested UDT browse via online type walker

Closes #315

docs/drivers/TwinCAT-Test-Fixture.md

@@ -57,6 +57,14 @@ All three gated on `TWINCAT_TARGET_HOST` + `TWINCAT_TARGET_NETID` env
 vars; skip cleanly via `[TwinCATFact]` when the VM isn't reachable or
 vars are unset.
 
+PR 4.1 / #315 adds `TwinCATUdtBrowseTests.Driver_browses_UDT_tree_and_flattens_to_atomic_leaves`
+which exercises `TwinCATDriver.DiscoverAsync` end-to-end against the
+`GVL_Plant` UDT fixture. Asserts the discovery surface emits one OPC UA
+variable per atomic leaf and folds `aAlarmRecords[1..2000]` into a
+single `IsArrayRoot` placeholder when the element count exceeds the
+default 1024-element cap (UDT per-member coverage; see
+`TwinCatProject/README.md §Complex hierarchy` for the supporting DUTs).
+
 ### Unit
 
 - `TwinCATAmsAddressTests` — `ads://<netId>:<port>` parsing + routing
@@ -66,6 +74,14 @@ vars are unset.
 - `TwinCATSymbolPathTests` — symbol-path routing for nested struct members
 - `TwinCATSymbolBrowserTests` — `ITagDiscovery.DiscoverAsync` via
   `ReadSymbolsAsync` (#188) + system-symbol filtering
+- `TwinCATTypeWalkerTests` — PR 4.1 / #315 nested-UDT decomposition:
+  atomic / single-level struct / nested struct / array-of-atomic
+  (in / over `MaxArrayExpansion`) / array-of-struct / alias chain /
+  pointer skip / self-referencing struct depth-cap / per-leaf
+  `MaxArrayExpansion` honored / ReadOnly propagation. Stub `IDataType`
+  / `IStructType` / `IArrayType` / `IMember` / `IDimensionCollection`
+  trees built in-test so the walker is exercised without
+  `Beckhoff.TwinCAT.Ads`-internal ctors.
 - `TwinCATNativeNotificationTests` — `AddDeviceNotification` (#189)
   registration, callback-delivery-to-`OnDataChange` wiring, unregister on
   unsubscribe