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.

art/sprites/FG_Tree_Summer.png.import

@@ -0,0 +1,40 @@
+[remap]
+
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+type="CompressedTexture2D"
+uid="uid://cepguvswk8ecj"
+path="res://.godot/imported/FG_Tree_Summer.png-1bc1b92f64d4677ada2e125e991dc553.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://art/sprites/FG_Tree_Summer.png"
+dest_files=["res://.godot/imported/FG_Tree_Summer.png-1bc1b92f64d4677ada2e125e991dc553.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1