Visual pass 2: tree + rock + stone wall sprite swaps

Replaces three procedural _draw() entities with bundle sprites:

- Tree: was draw_rect trunk + draw_circle canopy. Now Sprite2D using
  FG_Tree_Spring.png (64x80, 4 variants picked deterministically from
  tile coord). Bottom-anchored so trunk base sits at tile bottom, canopy
  rises into the cell above; y_sort_enabled so canopies tuck behind
  pawns south of the trunk. Chop-progress notch overlay retained.
- Rock: was draw_colored_polygon hex. Now Sprite2D reading from the
  existing FG_Grasslands_Spring.png decoration atlas at three eyeballed
  coords (2 gray boulders, 1 brown rock pile). Variant deterministic
  per tile. Mine-progress crack overlay retained.
- Stone wall: was procedural top-band + front-band + mortar lines. Now
  Sprite2D from FG_Fortress.png at (1,1) — clean tan-stone brick fill.
  Bottom-anchored (offset.y=-8) so the 16x16 sprite spans y=-16..0,
  matching the procedural draw box exactly. Ghost state via modulate.a.
  Wood walls still use procedural _draw_wood_wall — no clean 16x16 wood
  tile found in the bundle yet (Pixel Crawler Walls.png is 32x32, would
  need crop+rescale).

Asset additions:
- art/sprites/FG_Tree_Spring.png (Tier 1, Grasslands pack)
- FG_Fortress.png and FG_Grasslands_Spring.png were already in art/tiles
  from earlier passes; this commit just consumes them from new sites.

Headless boots clean, runtime verified: trees look like chunky pixel-art
trees with root flare, rocks read as real boulders, cabin walls show
proper brick texture.

License: all ElvGames Humble bundle — commercial OK with credit.
Credit-string compilation still open.

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

art/sprites/FG_Tree_Spring.png.import

@@ -0,0 +1,40 @@
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