Visual: sprinkle grass+flower decoration overlay (ElvGames bundle)

First visual pass: replaces nothing, adds a decoration TileMapLayer above
Terrain that paints ~8% of cells with random overlay sprites pulled from
FG_Grasslands_Spring.png. Mix of 3 grass-sprout variants, 3 white-with-
colored-center flowers, and 2 scattered-grass-dot patches. ~540 cells
populated on the 80x80 map; deterministic seed so layout is stable.

The bundle's design assumes flat-color terrain + overlay sprites for
visual richness — so this is the cheapest possible win that uses the
art we own. Terrain base, walls, trees, pawns, UI all unchanged.

Implementation lives in world.gd as _build_decoration_tileset() +
_paint_decorations(); the Decoration TileMapLayer is added to
world.tscn at z_index=0 (siblings render in tree order so it draws
between Terrain and Floor). Tileset is built at runtime pointing at
res://art/tiles/FG_Grasslands_Spring.png, mirroring the existing
_build_placeholder_tileset pattern.

Verified MCP runtime: world feels like a meadow now, no perf hit.
Headless boot logs '[world] decoration: painted 541 overlay cells'.

License: ElvGames Humble bundle — commercial use OK with credit (see
docs/art.md). Credit string compilation is still an open audit item.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

art/tiles/FG_Grasslands_Spring.png.import

@@ -0,0 +1,40 @@
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+}
+
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+dest_files=["res://.godot/imported/FG_Grasslands_Spring.png-e9676b030c6767775ad374c0db805fa9.ctex"]
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