fix(showcase): rebuild the grindstone so the A-frame and trough actually join

The trestle was two sticks with a hole under the bearing and a floating tub. King, nested sill, manifold basin, and shoe tenons close the joints, locked as hygiene and seat budgets.

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: TMHSDigital