fix(solver): enforce quaternion continuity on dragged parts during drag (#338)

The _enforce_quat_continuity function previously skipped dragged parts,
assuming the GUI directly controls their placement.  However, Newton
re-solves all free params (including the dragged part's) to satisfy
constraints, and can converge to an equivalent but distinct quaternion
branch.  The C++ validateNewPlacements() then sees a >91 degree rotation
and rejects the step.

Two-level fix:

1. Remove the dragged_ids skip — apply continuity to ALL non-grounded
   bodies, including the dragged part.

2. Add rotation angle check beyond simple hemisphere negation: compute
   the relative quaternion angle using the same formula as the C++
   validator (2*acos(w)).  If it exceeds 91 degrees, reset to the
   previous step's quaternion.  This catches branch jumps where the
   solver finds a geometrically different but constraint-satisfying
   orientation (e.g. Cylindrical + Planar with 180-degree ambiguity).

Verified: all 291 solver tests pass.

.mailmap

@@ -1,7 +0,0 @@
-forbes <contact@kindred-systems.com> forbes <joseph.forbes@kindred-systems.com>
-forbes <contact@kindred-systems.com> forbes <zoe.forbes@kindred-systems.com>
-forbes <contact@kindred-systems.com> forbes-0023 <joseph.forbes@kindred-systems.com>
-forbes <contact@kindred-systems.com> forbes-0023 <zoe.forbes@kindred-systems.com>
-forbes <contact@kindred-systems.com> josephforbes23 <joseph.forbes@kindred-systems.com>
-forbes <contact@kindred-systems.com> Zoe Forbes <forbes@copernicus-9.kindred.internal>
-forbes <contact@kindred-systems.com> admin <admin@kindred-systems.com>