Positive Energy

Which (if any) playable races exists natively on the positive plane other than Lumi?
Glimmerskins are technically playable, though with an ECL that high you wouldn't want to. You're asking an awful lot to even hope for "native to an energy plane," let alone a playable one. Vivacious creatures would also qualify and are an easier route, although that LA is ugly.

==== Undead and constructs are immune to Fortitude save-effects unless they work on objects too. A strongly positive-dominant plane offers a Fort save to avoid exploding, but specifies creatures. Can these two creature types get infinite HP on the Positive Energy Plane? ==== Positive energy on the Positive Energy Plane should affect objects as well, with the same dramatic results (glowing and exploding). This appears to be a rules oversight, and an unfortunate one. Positive energy should affect undead in the same way that negative energy affects the living, but of course, these books were written from the perspective of having a standard adventuring party, and don't do well at addressing issues of monstrous entities brought there. This is why, for example, ravids have no immunity to their home plane's effects and deal damage with their positive energy lash. I would recommend that DMs treat the Positive Energy Plane as hazardous to both undead and constructs, and not at all hazardous to ravids.