CABI: allow dropping a readable future end with a pending write

[rvolosatovs]

The Explainer specifies that a component may call future.drop-readable before reading a value to signal loss of interest, in which case a blocked future.write completes with future-write-result.dropped. But SharedFutureImpl.drop asserted that any pending buffer is a WritableBuffer, i.e. a pending read, while the only pending buffer actually possible at this point is the buffer of a pending write: a reader cannot drop its end while its own read is pending (CopyEnd.drop traps while copying) and a writer cannot drop its end before the write resolves (WritableFutureEnd.drop traps unless DONE). Exercising this spec-legal path failed the assert instead of notifying the writer with DROPPED.

Flip the assert to expect a ReadableBuffer (the source buffer of the pending write), make ReadableBufferGuestImpl actually subclass ReadableBuffer (it previously only derived BufferGuestImpl, unlike WritableBufferGuestImpl, which does mix in WritableBuffer), and add a test reproducing the scenario.