solidcraft-shell-and-tangent-arcs

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# solid.shell in SolidCraft: how it works, how it fails, how to get around it

## Syntax
`solid.shell {faces:[[x,y,z]], thickness, direction:"inside"|"outside"|"both", tangent_chain?, body?}`
- `faces` are points on the faces to REMOVE (open). For an underside cavity pick a point on the bottom face z=0 that lies inside the outline and outside bosses and bores, for example [-38,0,0].
- `direction:"inside"` keeps the outer shape. `body` names the target body.

## Failure 1: "offset: a corner whose faces don't meet the same way after the move"
- Cause: arcs that are meant to be tangent (for example a convex R12 around a hole boss and concave R75 side arcs) were entered with 5-decimal coordinates, so the faces are only nearly tangent. The offset treats the joints as real corners and fails for any thickness, even on the bare extruded base.
- Fix: compute the tangent points analytically (the point on the line between the two arc centres at distance r1 from the first centre) and enter the coordinates with 10 decimals in `sketch.arc.three_point` (start, end, through). The shell then works on the base and on base plus cylinder.
- `solid.fillet` on such an edge answers `edge is smooth` when the joint is truly tangent, so you can use it as a tangency test.
- `tangent_chain:true` did not help for this failure.

## Failure 2: shell hangs SolidCraft (minutes, needs a restart)