feat(v3-batch4): contracts — dual-namespace URIs, V3NodeIds, AddressSpaceRealm

Batch 4 contracts commit (coordinator lands before Wave A fan-out). Purely
additive so wave agents branch from a green base:

- AddressSpaceRealm enum (Raw | Uns) — the explicit realm discriminator that
  travels alongside a node's s= identifier at every sink seam (WP2 adds it to
  IOpcUaAddressSpaceSink / ISurgicalAddressSpaceSink; WP3 threads it).
- V3NodeIds — the two namespace URI constants (https://zb.com/otopcua/raw,
  https://zb.com/otopcua/uns, replacing the single .../ns) + Raw() pass-through
  (s= id == RawPath) + Uns()/UnsChild() slash-joined Area/Line/Equipment[/Eff]
  builders, both sharing RawPaths.Separator + ordinal segment validation.

Placement note: URIs live in V3NodeIds (Commons) rather than beside the retired
OtOpcUaNodeManager.DefaultNamespaceUri so runtime + tests reference them without
depending on OpcUaServer (single-source-of-truth, §5). WP2 rewires the node
manager to register both via V3NodeIds.RawNamespaceUri/UnsNamespaceUri.

EquipmentNodeIds retirement is NOT in this commit: its callers are the applier +
runtime (DriverHostActor/VirtualTagHostActor/DiscoveredNodeMapper), all WP3-owned
(the composer does not use it) — deleting it here would red the base. WP3 sweeps
it in Wave B.

Claude-Session: https://claude.ai/code/session_01LVneM3eh1UtJxEisFXgmox

src/Core/ZB.MOM.WW.OtOpcUa.Commons/OpcUa/AddressSpaceRealm.cs

@@ -0,0 +1,18 @@
+namespace ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+
+/// <summary>
+///     Which of the two v3 OPC UA namespaces a node lives in. Carried explicitly alongside a
+///     node's <c>s=</c> identifier at every address-space sink seam so the namespace is chosen at
+///     the call site, never parsed back out of the id string (explicit beats inferred).
+///     <see cref="V3NodeIds.NamespaceUri"/> maps each realm to its namespace URI.
+/// </summary>
+public enum AddressSpaceRealm
+{
+    /// <summary>The device-oriented Raw tree (Folder→Driver→Device→TagGroup→Tag); a node's
+    /// <c>s=</c> id is its <see cref="ZB.MOM.WW.OtOpcUa.Commons.Types.RawPaths">RawPath</see>.</summary>
+    Raw,
+
+    /// <summary>The equipment-oriented UNS tree (Area→Line→Equipment→signal); a node's <c>s=</c>
+    /// id is the slash-joined <c>Area/Line/Equipment[/EffectiveName]</c>.</summary>
+    Uns,
+}