[skin] v2: cavity fill, aperture closing, no-bridge -- from four real fights on rose-pro

[ecto]

Four workarounds rose-pro carries in pro/skin.py that belong in the operator (all reproducible on the rose pro/skin branch):

1. Cavity fill. A hollow bone group (channel + flanges) skins into a bag; we hand-author 'form block' filler cubes per panel. An SDF flood-fill of enclosed voids before the offset would delete that whole table.
2. Aperture closing below a threshold. The ball tunnels through any hole < 2r (thigh's 65x48 window at r 12 became a hole in the skin). We inflated r per panel to 26, which balloons the whole panel. 'Close apertures < N mm' separates the safety radius from topology.
3. No-bridge mode / per-pair exclusion. The closing bridged the two crossed hip actuator cylinders into X-shaped webs (rolling ball in the crotch of a cross). Correct math, wrong design outcome; we had to drop the actuators from the group entirely.
4. Coarsen-and-report instead of refuse past the grid cap (see eval: silent empty roots -- refusal paths must name the root, the cause, and the knob #827).

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