Procedural workbench redraws + 3-season tree variety

Workbenches: replace atlas sprites (which read as chest-of-drawers,
candle base, kitchen stove, cushion stack) with procedural _draw_ methods
following CremationPyre._draw_pyre's pattern. Carpenter shows a wood bench
with saw + log slabs; Smelter a stone furnace with smoking chimney; Hearth
a tall h=2 stone fireplace with arched opening + log fire; Millstone a
wood frame supporting a round grindstone wheel.

Trees: add Summer + Fall atlases alongside Spring (12 visual variants
from 4 silhouettes × 3 seasons). Selection hash mixes season independently
so neighbouring tiles don't all share the same palette.

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art/sprites/FG_Tree_Summer.png.import

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