Adds a random amount to the value in B, using 9BFD with a range of 10, and returns the result in A, clamped to 0-255.

The random amount C is a signed number between -10 and +10, but it is heavily biased towards positive values: only about 1 in 25 adjustments is negative, and +1 to +5 are twice as likely as +6 to +10 (see 9BFD).

BUG: the clamping is wrong for negative adjustments. After ADD A,B the code treats the carry flag as 'overflow'. When C is negative (say -3, which is $FD) and B is at least 3, the addition B+$FD always produces a carry, even though the true result (B-3) is perfectly valid. The code then sees that C is negative and returns 0 instead of B-3. So whenever the adjustment is negative, the value collapses to 0. When this happens to a defence value, the next blow is almost certainly fatal; when it happens to an attack value, the blow is wasted. This bug is the reason a bare-handed Bilbo occasionally kills Thorin, and one reason for the game's famous sudden deaths.