`EventStoreAuthoring` is public (#335).

A projected spec — an ordinary xUnit or TUnit test bound to a slice with `[BobcatSlice]`, which
0.24.0 shipped — could not arrange an event stream. The compiler's answer was CS0122. That made the
lane a good deal less useful than it looked: a test could be bound to a slice and rendered on the
canvas, but could not say "these events happened first", which is the arrange half of every
event-sourcing spec.

`CritterStackStepContextExtensions` is public, but every member hangs off an `IStepContext`, which
exists only inside a Bobcat run. `EventStoreAuthoring` is the one part of the store vocabulary
already free of the run — `AppendAsync` and `LoadDocumentAsync` take an `IEventStore` and nothing
else — and it was the one part sealed off. It was internal because the Gherkin fixture was the only
caller while the fixture was the only lane, not because there was a boundary there: `EventStores`,
`DocumentStores`, `TrackedActs` and `RecordBuilding` are all public in the same tier.

Found building CritterCrush's Lane A rebuild on 0.24.0, which is also what turned up the two
findings still open: #334, and the manifest's lack of a file-level authoring default.

17 assemblies, 1554 tests.