fix(ablegacy): stamp health from the injectable Clock; drop the redundant per-project TreatWarningsAsErrors (lmxopcua#768)

AbLegacyDriver stamped LastSuccessfulRead (init, read, write), the read's sample
timestamps and host-state change times from raw DateTime.UtcNow; only the backoff
path read the injectable Clock. All of them now read Clock(), so
NativePcccBridgeStatusHealthTests drives a manual clock (init at T0, read at T1)
and asserts LastSuccessfulRead == T1 exactly instead of spin-waiting for the wall
clock to pass the init stamp (#767). Without the driver change the 6 tests fail
(init stamp is wall-clock, not T0).

Directory.Build.props sets TreatWarningsAsErrors for every project since #766, so
the 75 per-csproj copies (64 src + 11 tests) are removed and the props comment
says it is now the only place. tools/mem-probe keeps its explicit opt-out.
Release build: 0 warnings, 0 errors; a planted CS0168 still fails the build in a
src project and in a test project that used to carry its own copy.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/ZB.MOM.WW.OtOpcUa.AdminUI.csproj

@@ -3,7 +3,6 @@
     <PropertyGroup>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.AdminUI</RootNamespace>
         <AddRazorSupportForMvc>true</AddRazorSupportForMvc>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
 
     <ItemGroup>

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/ZB.MOM.WW.OtOpcUa.ControlPlane.csproj

@@ -2,7 +2,6 @@
 
     <PropertyGroup>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.ControlPlane</RootNamespace>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
 
     <ItemGroup>

src/Server/ZB.MOM.WW.OtOpcUa.Host/ZB.MOM.WW.OtOpcUa.Host.csproj

@@ -4,7 +4,6 @@
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Host</RootNamespace>
         <AssemblyName>OtOpcUa.Host</AssemblyName>
         <UserSecretsId>zb-mom-ww-otopcua-host</UserSecretsId>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
         <!-- Microsoft.CodeAnalysis.CSharp.Scripting (4.12.0, pulled in via Core.Scripting for F8b
              user-script compilation) requires CodeAnalysis.Common 4.12.0 exactly, but ASP.NET
              Core's transitive Microsoft.CodeAnalysis.CSharp 5.0.0 wins resolution. Suppress

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/ZB.MOM.WW.OtOpcUa.OpcUaServer.csproj

@@ -2,7 +2,6 @@
 
     <PropertyGroup>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.OpcUaServer</RootNamespace>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
 
     <ItemGroup>

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/ZB.MOM.WW.OtOpcUa.Runtime.csproj

@@ -2,7 +2,6 @@
 
     <PropertyGroup>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Runtime</RootNamespace>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
 
     <ItemGroup>

src/Server/ZB.MOM.WW.OtOpcUa.Security/ZB.MOM.WW.OtOpcUa.Security.csproj

@@ -2,7 +2,6 @@
 
     <PropertyGroup>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Security</RootNamespace>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
 
     <ItemGroup>