[Foundation] FT-2: Introduce internal BuildContext + ambient activation #307

Summary

Introduce an internal BuildContext that owns per-run state, and activate it ambiently for the duration of a build. Foundation for de-statifying the engine (testability, reentrancy, parallel execution, and the plugin architecture in this milestone). No behavior change in this ticket — just stand up the context and make it active.

Why

The engine currently bootstraps per-run state into process-global statics in the FalloutBuild static ctor (FalloutBuild.Statics.cs:16-30), read directly throughout Execution/. That blocks isolation, safe parallelism, and a scoped plugin model.

Sketch

internal sealed class BuildContext
{
    public required FalloutBuild Build { get; init; }
    public required DirectoryConfig Directories { get; init; }   // Root, Temp, BuildAssembly…
    public required ParameterService Parameters { get; init; }
    public required ToolPathConfig Tools { get; init; }
    public required LoggingScope Logging { get; init; }
    public required HostInfo Host { get; init; }
    public IReadOnlyCollection<IBuildExtension> Extensions { get; init; }

    private static readonly AsyncLocal<BuildContext> s_current = new();
    public static BuildContext Current => s_current.Value ?? throw …;
    public static IDisposable Activate(BuildContext ctx); // set + restore
}

BuildManager.Execute builds the context and wraps the run in using (BuildContext.Activate(ctx)).

Acceptance criteria

• BuildContext is constructed in BuildManager.Execute and active for the whole run.
• All existing tests still pass (public statics untouched in this ticket).

Notes

• Non-breaking. Internal only (stays internal per milestone policy).
• Depends on: FT-1.
• Blocks: FT-3, FT-4, FT-5, FT-6, FT-7, FT-8.
• Size: L.