Horizontal and vertical distance dimensions

Port of snaporca 9fa304c77a. Parity OK: 17 files identical, 8 diverging at their
expected counts.

The everyday dimension in SolidWorks and Onshape, and this kernel had no form of
it. Distance constrains the straight-line gap; LockX/LockY pin one point's
ABSOLUTE coordinate. Neither relates two points along an axis.

DistanceX/DistanceY emit SLVS_C_PROJ_PT_DISTANCE against two unit direction
lines built in the solver's G_FIXED group, so they add no degrees of freedom.

THE DIRECTION IS SIGNED, and getting it backwards is silent. libslvs defines a
LINE_SEGMENT's direction as point[0] - point[1] (entity.cpp) and
PROJ_PT_DISTANCE constrains (pB - pA).dot(dir) (constrainteq.cpp:234), so the
reference lines are built head-first to mean +X and +Y.

The same signedness was a real defect in the GUI: the inline editor was
pre-filled with |delta|, so when the closest endpoint pair ran right-to-left,
opening the dimension and accepting the number shown would flip the point to the
other side of its anchor. Opening a dimension and accepting its own value must
be a no-op. The refs are now ordered so the shown value is positive.

On the CAD-1000-hours corpus, dimensioning and constraining is 31.9% of all
observed CAD time -- the largest single class, 7.6x feature operations. This is
the item in the constraint epic that lands most directly on it.

VERIFICATION LIMIT, as with the previous commit: this fork's kernel suite still
cannot run (find_package(assimp) fails at configure time, snaporca-w80c). The
shared sources are byte-identical to snaporca's, where kernel is 2603 assertions
/ 200 cases and ALL LADDERS HELD across all seven rungs.

resources/images/design_c_dist_x.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#b6b6b6" stroke-width="0.85" stroke-linecap="round" stroke-linejoin="round"><path d="M6 6v12M18 6v12M6 12h12M6 12l2.5-2M6 12l2.5 2M18 12l-2.5-2M18 12l-2.5 2"/></svg>

resources/images/design_c_dist_y.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#b6b6b6" stroke-width="0.85" stroke-linecap="round" stroke-linejoin="round"><path d="M6 6h12M6 18h12M12 6v12M12 6l-2 2.5M12 6l2 2.5M12 18l-2-2.5M12 18l2-2.5"/></svg>