Tree growth: dedicated stage atlas + tuned WildGrowth rate

Sub-mature stages (0/1/2) now use FG_Tree_Stages.png (a 128×32 atlas
with 8 progressively-larger tree cells from the bundle's "Crops with
Stages 03" pack). Stage 0 = tiny sprout (col 0), Stage 1 = small
leaf (col 1), Stage 2 = small tree (col 3). Stage 3 (Mature) keeps
the existing 64×80 seasonal canopy atlases.

Visually distinct progression replaces the previous scale-down-the-
mature-texture placeholder + procedural sapling dots.

WildGrowth pacing tuned: INTERVAL 1200 → 3000, PROBABILITY 0.30 →
0.12, LIMIT 60 → 80. Previous values flooded the map with saplings
within ~30 seconds of 12× play. New rate gives a slow but visible
regrowth over a season at default speed.

_draw simplified: removed procedural sapling fallback (atlas handles
all stages now). Pending-plant ghosts get the alpha tint via
sprite.modulate.

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