Enhance limb healing functionality

[Venera3]

Summary

None

Purpose of change

Limbs need some more options to mess with their healing.

Describe the solution

• Modified the BP healing logic slightly to only run when the healed amount would be different from 0
• Added two new bodypart flags: HEAL_OVERRIDE replaces the healing step of the limb with the bodypart's heal_bonus, while ALWAYS_HEAL triggers the bonus on every regen roll not just successful ones

Describe alternatives you've considered

Not mess with the logic, but it kind of messed with what I intended a negative heal_bonus to be (and running that whole thing every five minutes for every BP felt a bit...messy).

Testing

Tried all combos: Base healing is unchanged, Debug Tail heals up fast (heal_bonus of 15, no flag changes), with ALWAYS_HEAL it's up to full health in 10 minutes, with HEAL_OVERRIDE on top minimally slower.

Additional context

This should be useful for e.g. bionic/prosthetic limbs that don't heal and makes "wasting" limbs less of a gutpunch.