Impairment penalty as a non-positive number (0 when none). The worst
(most negative) of the permanent impairment and each current injury.
Display status bucket for the header grid (#464).
Impairment tier by magnitude (drives the health ceiling, #470).
Whether the part can still be used. false when it has a grievous
injury or is flagged permanentlyUnusable. Permanent impairment
(however severe) never makes a part unusable.
The derived impairment of a single body part.