Fix union()/difference()/intersection() dropping all geometry when one operand is invalid

[revarbat]

Summary

• generateCsg fed every foreground body directly into Manifold's +/-/^ boolean operators. Manifold propagates a non-NoError Status() (e.g. NonFiniteVertex, typically from a degenerate accumulated transform somewhere upstream) onto the whole combined result instead of treating that one operand as an empty no-op contributor — so a single already-invalid part silently zeroed out every valid sibling in the same union().
• Found via a real user project (snappy-reprap): an entire ~65-part extruder assembly, positioned deep inside several generations of indexed children()-forwarding, vanished from the render with no warning at all — while the exact same assembly rendered fine called directly/shallowly. Real OpenSCAD's CGAL backend doesn't hit this on the same input.
• Fix: filter out any operand whose own Status() isn't NoError before handing the group to Manifold's boolean ops, matching this evaluator's existing "invalid/empty operand doesn't poison the whole op" precedent for genuinely-empty statements.

Test plan

• New regression test (Union.OneInvalidOperandDoesNotDiscardValidSiblings) reproduces the exact mechanism with a deliberately NaN-vertex polyhedron() unioned alongside a valid cube() — confirmed it fails on the pre-fix code (volume 0 instead of 8) and passes after the fix
• Full 747-test differential suite passes both OSCAD_BYTECODE_VM=0 and =1
• Re-ran real user project snappy-reprap/full_assembly.scad: the extruder assembly (a real, non-degenerate 22,895-vertex merged body) now appears; body count 507→508, warnings unchanged at 2 (pre-existing/unrelated)

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