[Foundation] Introduce DI container in build orchestrator

[ChrisonSimtian]

Part of the v11 internal-foundation workstream. See docs/roadmap.md and milestone #6.

Goal

Introduce Microsoft.Extensions.DependencyInjection inside the build orchestrator. Set up a composition root that registers default implementations of every infrastructure interface (file system, process runner, console host, etc.) and resolves them via constructor injection.

Consumer-facing API stays identical — Execute<T>() is still the static entrypoint; internally it builds the ServiceProvider, registers defaults, runs the orchestrator.

Scope

• Add Microsoft.Extensions.DependencyInjection to Directory.Packages.props.
• Create a BuildHostBuilder that produces a configured IServiceProvider per build invocation. Lives in Fallout.Build.
• Register defaults for the interfaces from [Foundation] Wrap IO/process/console statics behind injectable interfaces #91 — IFileSystem, IProcessRunner, IConsoleHost, IBuildStatePersistence.
• Plumb the IServiceProvider through BuildExecutor, ExecutionPlanner, ParameterService. Replace direct static calls with resolved dependencies.
• Keep static facades (ProcessTasks.StartProcess, AbsolutePath.*) as default wrappers that resolve their dependency from a per-build ambient context (AsyncLocal<IServiceProvider> or similar). Statics on the surface, DI underneath.

Verification

• BuildExecutor unit tests can now pass a fake IProcessRunner and assert it was invoked as expected. Add at least one as proof.
• ./build.ps1 still runs end-to-end with no behavioral changes.
• No IServiceCollection.AddSingleton<T> call reaches into Fallout.Common internals — registration is shaped against the interfaces only.

Out of scope (tracked separately)

• The interfaces themselves (IFileSystem etc.) — [Foundation] Wrap IO/process/console statics behind injectable interfaces #91.
• Replacing the static BuildManager with an instance type — [Foundation] Replace BuildManager statics with instance BuildOrchestrator #90.
• Exposing DI registration as a public plugin extension point — v12.

Blocks

• [SDK-7] Promote IBuildMiddleware / IBuildOrchestrator from internal to SDK-public #157 — [SDK-7] Promote IBuildMiddleware / IBuildOrchestrator from internal to SDK-public

[ChrisonSimtian]

Superseded by the [Foundation] FT-* series (epic #315), which re-decomposes this engine-foundation work with file:line findings and an explicitly backwards-compatible, facade-preserving scope (vs. the obsolete v11 "breaking" framing here). Tracked going forward by: #307 (BuildContext) + #308 (facades) — the lightweight ambient-context approach replaces a full DI container. Closing to avoid double-counting milestone #6.