The seventh operation: `cube(2)` gains CUBE_OP_RANGE, a bounded walk of one
space's records that lie in a box. Its argument block is its own (`cube_range_args`,
versioned by `size` like the walk's), and the box travels as its six numbers for
the reason a coordinate does — the key it has to become is the driver's business.
The operation rests on the span that the shared file already owns. `key_span(lo, hi)`
bounds every key in the box because the interleave is monotone on each axis, so a
v3 image is **sought**: the fixed-stride index is binary-searched for the span's foot
(`Addressed::lower_bound`) and read forward to its head, merging the log's edits
exactly as a walk does. A packed v1/v2 image has no index to search, so its space is
walked with the same span used only to stop early — and the contract is the same
either way, so a caller is not told which path it got.
The trap that shaped the code, and the reason it is written the way it is: **the span
is a bound, not the set.** Keys of points outside the box fall inside it (Z-order
amplification), so every candidate is decoded and tested against the box before it
is returned — which is what `morton_decode`, the interleave's inverse, is for, now in
the shared file with the same kind of hand-pinned tests the interleave has. And the
cursor counts the records *in the box*, not the records of the space, because those
are the records the walk returns.
The over-coverage — records examined versus records returned — is the number this
operation is meant to publish, and it is not wired to the caller yet: the cost gate
measures it by comparing against the userspace store, which counts the same thing.
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