Stat calculation tweak, higher evasion in Guard

Calculated stat values now have a dynamic range of 4--in other words,
the final value will range between 25 and 100.  By contrast, the old
formula put the maximum value between 50 and 100, a dynamic range of
only 2.

For Guard, the stance now raises evasion by 50% for melee attacks, up
from the original 25%.

Also fixed the Headless Horse AI to work with the refactored AIContext.