Equal radius and Collinear, and one Equal button that knows what it picked

Port of snaporca 9ec6405e2d. Parity OK: 17 files identical, 8 diverging at their
expected counts (DesignPanel.cpp 32, test_slvs_constraints.cpp 3).

Measured on the CAD-1000-hours corpus: 51.5% of observed CAD time is 2D sketch
work, and dimensioning/constraining alone is 31.9% -- the largest single class.
Two constraints every industrial sketcher has were missing here.

EqualRadius fixes a dead end rather than adding a feature. Picking two circles
and pressing Equal emitted EqualLength, which maps to SLVS_C_EQUAL_LENGTH_LINES
and constrains nothing on a curve: a silent no-op with no error. Equal is now
one button with two meanings, as in Onshape and SolidWorks.

Collinear emits PARALLEL plus PT_LINE_DISTANCE=0 rather than PT_ON_LINE, whose
internal valP param this libslvs port leaves at 0, drifting an already-collinear
pair.

Both types are appended at the END of SketchConstraintType: cereal serializes it
positionally, so inserting elsewhere reinterprets every saved recipe.

VERIFICATION LIMIT, stated rather than implied: this fork's kernel suite could
NOT be run. scripts/CAD/run-kernel-tests.sh fails at CMake configure time on
find_package(assimp), before any source compiles -- a pre-existing deps gap
(snaporca-w80c), not this change. The shared sources are byte-identical to
snaporca's, where the full gate passed: kernel 2588/195 and ALL LADDERS HELD
across all seven rungs.

Also fixes two defects in this fork's scripts/CAD/run-all-checks.sh:
  - `cd $(dirname $0)/..` landed in scripts/ instead of the repo root, so every
    rung looked for itself under scripts/scripts/. Broken since the script moved
    into scripts/CAD/; the three sibling scripts were fixed then and this was
    missed, so the gate has not run since.
  - C defaulted to snaporca-gui, the OTHER fork's rig container, so this fork's
    gate would drive snaporca's app and report green about the wrong binary.
    run-kernel-tests.sh:31 documents the identical defect being fixed once
    already for the build volume; this is the third instance.

resources/images/design_c_collinear.svg

@@ -0,0 +1 @@
+<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="M2 12h20" stroke-dasharray="1.5 1.5"/><path d="M5 12h5M14 12h5"/></svg>

resources/images/design_c_equal_radius.svg

@@ -0,0 +1 @@
+<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"><circle cx="4" cy="12" r="2.5"/><circle cx="15" cy="12" r="4.5"/><path d="M8 10h2M8 14h2"/></svg>