The test suite is now **green** again.
The design rework and clean-up of unnecessary test-driven functionality
in the `DiagnosticBufferProvider` and `DiagnosticOutputSlot` required
to break some very old test-setup, using fake instances of the relevant
context facilities. This setup however was used by the newer family
of test support facilities, notable the `MockJob`, `MockSegmentation`
and `MockDispatcher`. These required a set of `DataSink` handles to be
present (even while they are not used currently).
This could be fixed easily now, relying on the simplified internal structure
of the `OutputSlot`: by re-arranging the types, the implied identity of the
`DataSink` handles could be made explicit, allowing for equality comparison.
__Remark__: even while `DataSink` with only a single abstracted function,
using function notation turned out to be rather confusing at usage site.
Naming this function as `lockFrame(frameNo)` clarifies its purpose nicely.
That call is effectively abstracted by means of the ''buffer type'',
relying on the VTable in the `BufferProvider` implementation.
Within the reworked design of the `OutputSlot` a proxy implementation
of the `BufferProvider` API is used (class `OutputBufferProxy`).
After analysis of the invocation patterns, I concluded that it seems adequate
to attach the special-purpose metadata generated by this integration
to the generic system of thread-local replicated `BufferMetadata` storage.
However, since thread-local instances are effectively static(global),
I was concerned that unit-testing might be impeded by accumulation of
stale metadata in the store linked to the main thread. Thus I created
a special configuration variant to retain the previously used setup
for the sake of unit testing (which applies especially to use of the
`DiagnosticOutputSlot`). Using a `SimpleBufferStateRegistry` for in
that case has the benefit that simply a local instance can be used
in the test and all metadata will be discarded afterwards.
Furthermore the factory syntax for obtaining an `OutputSlot` was improved,
drawing on the recent changes to `lib::ScopedCollection`.
Also I looked through some of the documentation and updated
the UML sequence diagram to reflect `OutputSlot`'s improved internal structure
