test(abcip): Emulate-tier nested-UDT live-gate smoke + docs (backlog #6)

Add AbCipEmulateNestedUdtTests (skip-gated, AB_SERVER_PROFILE=emulate) to close
the live-gate gap for nested-struct UDT discovery via CIP Template Object (class 0x6C)
threaded by commits 3d8ce4e8/d203f31c. Compiles + skips cleanly against ab_server
(no CIP Template Object service). Update docs/drivers/AbCip.md nested-struct section
to record the shipped decode path, the Emulate-only live-gate, and offline unit coverage.

docs/drivers/AbCip.md

@@ -132,13 +132,23 @@ Variables — in addition to the UDT container tag itself.
   separate Variable node under the UDT's `Discovered/` sub-folder, addressable by its member
   path (e.g. `MyUdt.Temperature`, `MyUdt.Flags`). Top-level atomic member discovery is
   **functional in production**.
-- **Nested struct members** — when a UDT member is itself a struct, the driver walks into it up
-  to a **depth cap of 8 levels**. Nested-struct expansion is a **documented deferral in
-  production**: the Template Object member block carries no nested template ID, so the
-  sub-shape cannot be re-fetched from the controller at discovery time. Nested struct leaves are
-  never mis-emitted — they are simply dropped (the parent member is omitted from discovery if it
-  is not atomic and its sub-shape is unavailable). Use pre-declared `Members` for nested structs
-  that must be individually addressed.
+- **Nested struct members** — **shipped** (commits 3d8ce4e8 / d203f31c). When a UDT member is
+  itself a struct, the Template Object member block carries the nested UDT's template instance ID
+  in the low 12 bits of the member `info` field (struct flag `0x8000` set). The driver captures
+  this as `AbCipUdtMember.NestedTemplateId` and threads it into a second `FetchUdtShapeAsync`
+  call (`@udt/{id}`) so the nested struct's atomic leaves are expanded into dot-joined addressable
+  paths (e.g. `Motor1.Status.Code`, `Motor1.Status.Running`). Recursion is bounded by a
+  **depth cap of 8 levels**; members at or beyond the cap are dropped rather than emitted.
+  Nested struct leaves that resolve to no emittable atomic leaf produce no sub-folder (lazy
+  materialisation).
+
+  **Live verification is Emulate-tier only** (`AB_SERVER_PROFILE=emulate` + a Logix Emulate
+  instance): `ab_server` (the default Docker simulator) does not implement the CIP Template
+  Object service (class 0x6C), so the nested-id fetch cannot be exercised against it.
+  See `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.IntegrationTests/Emulate/AbCipEmulateNestedUdtTests.cs`
+  for the skip-gated live-gate smoke (backlog #6). Offline unit coverage lives in
+  `AbCipDriverDiscoveryTests.Controller_discovered_UDT_nested_struct_expands_via_nested_template_id_fetch_no_seam`
+  and `CipTemplateObjectDecoderTests`.
 
 ### Bare-container reads