Add transparent enemy unit art

docs/DATA_MODEL.md

@@ -219,7 +219,7 @@ Classes provide movement, min/max attack range, skill access, growth grades, cla
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 `attack_range` is `[min, max]` in Manhattan distance. A range of `[2, 2]` means the unit cannot attack adjacent targets.
-`sprite` is an optional class-level board token image used by hydrated units unless an officer or deployment overrides it.
+`sprite` is an optional class-level board token image used by hydrated units unless an officer or deployment overrides it. `enemy_sprite` is an optional class-level image for generic enemy units with `team: "enemy"` and no `officer_id`; runtime sprite precedence is deployment `sprite`, officer `sprite`, generic enemy class `enemy_sprite`, then class `sprite`. The current enemy class art is stored as transparent PNG cutouts under `res://art/units/enemies/` so named officers and player units can keep their cleaner class or officer presentation while ordinary troops read as their own force.
 
 `promotion.level` and `promotion.to` define an automatic class promotion route. When a unit reaches the threshold during battle, the runtime updates class name, movement, movement type, growth, class skills, attack range, and class base-bonus deltas. The promoted `class_id` and derived roster fields are captured in the campaign roster snapshot, and the next battle rehydrates class-derived fields from the saved `class_id`. Promotion targets must be higher-tier classes and must keep all source equipment slot permissions unless runtime auto-unequip logic is added later. The current core routes cover hero, cavalry, infantry, archer, warrior, and strategist first-tier classes.