- _flanking_bonus checks for an active same-side ally with a melee weapon
at the position diametrically opposite the attacker through the target.
- +2 applies to melee attack rolls and crit-confirm rolls; ranged attacks
ignore flanking; allies without melee weapons don't threaten.
- AttackResult gains flank_bonus field; transcript shows '+2(flank)' when
active.
- 6 new tests: opposite ally grants +2, no ally = no bonus, ranged ignores
flanking, diagonal flanking, downed ally doesn't flank, enemy on opposite
side doesn't flank.
- README and module docstring updated; test count 196 -> 202.
ctan
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