Tutorial: Old Tam's hut outline beside the fire, pieces snap into it

docs/TUTORIAL_DESIGN.md

@@ -27,7 +27,13 @@ real worlds) are proposals.
 - tests/test_tutorial_full.gd: a bot plays all 22 steps through Actions,
   in order, to the card. (Bulk materials, like a hut's wood, go straight
   into its bag; every goal is done as a player does it.)
-- Not yet: a ghost outline for the hut (§2 step 17), sounds for the tick.
+- The hut outline (§2 step 17): Old Tam marks out a 20-piece hut, pale
+  as a ghost, on the first clear flat spot 5 m or more from your fire
+  (scripts/ui/hut_outline.gd). A piece held within 2 m of its matching
+  ghost snaps right into it; each ghost fades as it's filled, and the
+  marker points at the outline. Building elsewhere still counts. The
+  outline goes when the step ends or the tutorial is left.
+- Not yet: sounds for the tick.
 
 *As built in T1:*
 - Actions has signals for what players do: `crafted`, `harvested` (with