Sign.proj and Tools.proj unconditionally require both WiX 3 and WiX 5 toolsets

## Summary

`Sign.proj` and `Tools.proj` unconditionally reference both WiX 3 (`Microsoft.Signed.Wix`) and WiX 5 (`Microsoft.WixToolset.Sdk`) toolset packages, forcing **every** repo using Arcade SDK 10 to download and validate both — even repos that produce no MSI/WiX installers at all.

This was introduced by PR #15962 ("Wix 5 signing support", merged 2025-07-14).

## Repro

Any repo onboarded to Arcade SDK `10.0.0-beta.*` (e.g. a NuGet-only repo with no WiX usage) will restore both WiX packages and `SignToolTask` will validate both tool paths exist.

## Root Cause

Three files contribute to the problem:

### 1. `DefaultVersions.props` — unconditionally sets both versions

```xml
3.14.1-9323.2545153
5.0.2-dotnet.2811440
```

### 2. `Tools.proj` — unconditionally downloads both packages (no opt-in gate)

```xml

```

Compare with `Microsoft.DotNet.Build.Tasks.VisualStudio` which is gated behind `UsingToolVSSDK`:

```xml

```

### 3. `Sign.proj` — unconditionally passes both tool paths to `SignToolTask`

```xml
Wix3ToolsPath="$(WixInstallPath)"
WixToolsPath="$(NuGetPackageRoot)microsoft.wixtoolset.sdk\$(MicrosoftWixToolsetSdkVersion)\tools\net472\x64"
```

### 4. `SignToolTask` — validates both paths and errors if set but missing

```csharp
if (Wix3ToolsPath != null && !Directory.Exists(Wix3ToolsPath))
Log.LogError($"Wix3ToolsPath ('{ Wix3ToolsPath}') does not exist.");
if (WixToolsPath != null && !Directory.Exists(WixToolsPath))
Log.LogError($"WixToolsPath ('{ WixToolsPath}') does not exist.");
```

## Impact

- Repos that only produce NuGet packages (no MSIs/wixpacks) are forced to download ~100MB+ of WiX tooling they never use
- If WiX packages fail to restore (feed issues, network), signing breaks even though no WiX artifacts exist
- The wixpack files used to zip and sign MSIs differ between WiX 3 and WiX 5, and `SignToolTask` expects both toolsets to be present even if only one version is in use

## Suggested Fix

Gate WiX toolset references behind an opt-in property (similar to `UsingToolVSSDK`):

```xml

```

```xml

Wix3ToolsPath="$(WixInstallPath)" Condition="'$(UsingToolWix3)' == 'true' or '$(UsingToolWix)' == 'true'"
WixToolsPath="..." Condition="'$(UsingToolWix5)' == 'true' or '$(UsingToolWix)' == 'true'"
```

Or at minimum, only pass the tool paths when the corresponding version property is explicitly set by the repo (not just defaulted).

## Environment

- Arcade SDK: `10.0.0-beta.26168.104`
- Source: dotnet/dotnet VMR @ `73b3b5ac0e4a5658c7a0555b67d91a22ad39de4b`
- Affected repo: dotnet/maui-labs (NuGet packages only, no WiX installers)

/cc @NikolaMilosavljevic @joeloff