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.

src/libslic3r/CAD/SketchEngine.hpp

@@ -84,7 +84,11 @@ enum class SketchConstraintType {
     Tangent, Midpoint, Symmetric, Angle,
     Radius, Diameter,
     PointOnLine,  // a point lies on a line (or at signed perpendicular distance `value`)
-    PointOnObject // a point lies on an entity edge (line -> PT_ON_LINE, circle -> PT_ON_CIRCLE)
+    PointOnObject, // a point lies on an entity edge (line -> PT_ON_LINE, circle -> PT_ON_CIRCLE)
+    // Append-only: cereal serializes this enum positionally as its underlying int, so
+    // inserting anywhere but the end reinterprets every constraint in every saved recipe.
+    EqualRadius,
+    Collinear
 };
 
 // Constraint on a SketchProfile, referencing profile point indices (a,b,c,d).

src/libslic3r/CAD/SketchSolver.cpp

@@ -173,6 +173,9 @@ static SketchSolveResult solve_system(std::vector<SketchEntity>& entities,
             ref_ok = ptOf(c.ea, c.ra) && ptOf(c.eb, c.rb) && primOf(c.ec); break;
         case CT::PointOnLine: case CT::PointOnObject:
             ref_ok = ptOf(c.ea, c.ra) && primOf(c.eb); break;
+        case CT::EqualRadius:
+        case CT::Collinear:
+            ref_ok = primOf(c.ea) && primOf(c.eb); break;
         }
         if (!ref_ok) continue;
         switch (c.type) {
@@ -280,6 +283,20 @@ static SketchSolveResult solve_system(std::vector<SketchEntity>& entities,
             else
                 b.C(SLVS_C_PT_ON_LINE, 0, ptOf(c.ea, c.ra), 0, primOf(c.eb), 0);
             break;
+        case CT::EqualRadius:
+            b.C(SLVS_C_EQUAL_RADIUS, 0, 0, 0, primOf(c.ea), primOf(c.eb));
+            break;
+        case CT::Collinear:
+            // libslvs has no collinear code. Two lines are collinear iff they are parallel
+            // AND a point of one lies on the other's infinite line — emit both.
+            b.C(SLVS_C_PARALLEL, 0, 0, 0, primOf(c.ea), primOf(c.eb));
+            // Point-on-infinite-line via PT_LINE_DISTANCE=0 rather than PT_ON_LINE: the
+            // latter creates an internal `valP` param that this port's Slvs_Solve leaves at
+            // 0 in the working set (ModifyToSatisfy only updates SK.param), so an already
+            // collinear pair drifts. PT_LINE_DISTANCE=0 is the same condition with no extra
+            // parameter, so an already-satisfied solve is a clean no-op.
+            b.C(SLVS_C_PT_LINE_DISTANCE, 0, ptOf(c.eb, Role::P0), 0, primOf(c.ea), 0);
+            break;
         }
     }