[Design] PluginSystemOptions.RegisteredALCs is hidden mutable state — side-channel coupling between PluginLoader and PluginManager

## Summary

`PluginSystemOptions` contains an `internal` mutable list of `AssemblyLoadContext` instances (`RegisteredALCs`). This list is populated by `PluginLoader` and consumed by `PluginManager`. Using a public options/configuration object as a hidden communication channel between two components is an anti-pattern: the options class should carry only configuration values, not mutable runtime state.

## Location

`Plugins/FluentCMS.Infrastructure.Plugins/PluginSystemOptions.cs` — line 51

## Problematic Code

```csharp
internal List RegisteredALCs { get; } = [];
```

## Impact

- **Hidden coupling:** `PluginLoader` and `PluginManager` are implicitly coupled through the shared options object with no documented contract.
- **Unexpected mutation:** Any component that holds a reference to `PluginSystemOptions` can observe or modify the ALC list, which should be an implementation detail.
- **Testing difficulty:** Creating a `PluginSystemOptions` instance in tests doesn't give the expected clean state without knowing about this hidden list.
- **Violation of SRP:** `PluginSystemOptions` now serves two roles: configuration data-bag and runtime ALC registry.

## Recommendation

Remove `RegisteredALCs` from `PluginSystemOptions` and change `LoadPluginTypes` to return a result type that packages both outputs together:

```csharp
public record PluginLoadResult(
IReadOnlyList PluginTypes,
IReadOnlyList LoadContexts
);

// PluginLoader.LoadPluginTypes() returns PluginLoadResult
// PluginManager receives PluginLoadResult directly
```

This makes the data-flow explicit and removes the hidden state from the configuration object.

## Severity

🔵 **Design / Maintainability**

Start with Plugins/FluentCMS.Infrastructure.Plugins/PluginSystemOptions.cs at the RegisteredALCs property, then trace PluginLoader.LoadPluginTypes and the PluginManager call sites that consume the list. Define the result shape described in the issue, pass it directly between the components, and remove the runtime registry from the options object. Done means the data flow is explicit and existing plugin behavior remains intact.

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