Optimize bag_distance by calling bag_difference only once

[preciz]

Assisted by: Antigravity CLI : Gemini Flash 3.5

Provides around 25% speedup for short inputs.

String.bag_distance/2 only needs one bag-difference pass.

The old code computed:

diff1 = bag_difference(bag1, bag2)
diff2 = bag_difference(bag2, bag1)
max(diff1, diff2)

For each grapheme count pair {count1, count2}:

max(count1 - count2, 0) - max(count2 - count1, 0) == count1 - count2

Summing over the bags gives:

diff1 - diff2 == length1 - length2
diff2 == diff1 - length1 + length2

So max(diff1, diff2) can be derived from diff1 and the two lengths:

if length1 >= length2, do: diff1, else: diff1 - length1 + length2

This preserves the exact same numerator passed to the final distance calculation while avoiding the second bag_difference/2 traversal.

[josevalim]

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