Workbench sprites — swap procedural draw for tileset art per variant

- Carpenter → FG_Interior (24, 20) 16×32 wood cabinet w/ drawers
- Smelter   → FG_Marketplace (8, 30) anvil + hot metal
- Hearth    → FG_Interior (16, 32) stove w/ burners
- Millstone → FG_Interior (17, 40) wood barrel + procedural wheel overlay

Adds label_text setter so the sprite rebuilds idempotently whether the
caller assigns label before or after setup() (world.gd assigns after,
SaveSystem after). Setter also calls _maybe_build_light() to fix a
pre-existing Phase 11 bug where Hearth never built its PointLight2D
(label_text was still default when _ready fired).

Unrecognised label_texts fall through to _draw_generic so ad-hoc
workbench variants keep rendering.

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

art/tiles/FG_Marketplace.png.import

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