[SDK-1] Create Fallout.Plugin.Sdk package skeleton

Bootstrap the public abstractions package the plugin ecosystem compiles against.

## Scope

- New project `src/Fallout.Plugin.Sdk/Fallout.Plugin.Sdk.csproj`.
- Sole runtime dep: `Microsoft.Extensions.DependencyInjection.Abstractions` (per [RFC #99](https://github.com/ChrisonSimtian/Fallout/issues/99)).
- Public API baseline file (`PublicAPI.Shipped.txt` / `PublicAPI.Unshipped.txt` via `Microsoft.CodeAnalysis.PublicApiAnalyzers`).
- Multi-targets matching the rest of the framework.
- Pack as `Fallout.Plugin.Sdk` on nuget.org (NuGet prefix already reserved — see memory: project_nuget_prefix_gotcha).
- Tests project `tests/Fallout.Plugin.Sdk.Tests`.

## Depends on

v11 foundation: #88 (Fallout.Core extraction), #89 (DI container).