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/Client/ZB.MOM.WW.OtOpcUa.Client.CLI/ZB.MOM.WW.OtOpcUa.Client.CLI.csproj

@@ -4,7 +4,6 @@
         <OutputType>Exe</OutputType>
         <TargetFramework>net10.0</TargetFramework>
         <Nullable>enable</Nullable>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
         <ImplicitUsings>enable</ImplicitUsings>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Client.CLI</RootNamespace>
     </PropertyGroup>

src/Client/ZB.MOM.WW.OtOpcUa.Client.Shared/ZB.MOM.WW.OtOpcUa.Client.Shared.csproj

@@ -3,7 +3,6 @@
     <PropertyGroup>
         <TargetFramework>net10.0</TargetFramework>
         <Nullable>enable</Nullable>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
         <ImplicitUsings>enable</ImplicitUsings>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Client.Shared</RootNamespace>
     </PropertyGroup>

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/ZB.MOM.WW.OtOpcUa.Client.UI.csproj

@@ -4,7 +4,6 @@
         <OutputType>WinExe</OutputType>
         <TargetFramework>net10.0</TargetFramework>
         <Nullable>enable</Nullable>
-        <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
         <ImplicitUsings>enable</ImplicitUsings>
         <RootNamespace>ZB.MOM.WW.OtOpcUa.Client.UI</RootNamespace>
     </PropertyGroup>