In quarry, skip past undiggable nodes

The quarry used to get stuck when it encountered an undiggable node.
Change it to skip past that node, digging whatever later stuff it can.
Necessarily, the current digging position becomes semantically-significant
state: it is no longer sufficient to search the quarry cuboid from the top
on each iteration.  The current digging height is reported in the quarry's
interaction form, and can be reset to the top using a button on the form.

Where there is a non-air node within the quarry directly above the
next node to dig, it blocks the quarry's access to that node, even if
everything involved is diggable.  Thus an undiggable node casts a shadow
of undug nodes below it.  Resolving undiggability of a node is a major
reason to use the restart button.

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