fix sapling auto-growth; link calculator in tutorial

DriftersUserDefined/pancelor-debug/DeadTree.gd

@@ -11,7 +11,7 @@ func evolve():
 		intend_transmute("res://DriftersUserDefined/pancelor-debug/Flames.tscn")
 
 func tweak():
-	if randf()*50<1:
+	if randf()*100<1:
 		world.log("an unlucky spark starts a fire")
 		intend_transmute("res://DriftersUserDefined/pancelor-debug/Flames.tscn")
 	else:

DriftersUserDefined/pancelor-debug/River.gd

@@ -1,8 +1,13 @@
 extends Drifter
 
+# every so often
+func evolve():
+	tweak()
+
 # when the player clicks
 func tweak():
 	if randf()*20<1:
 		intend_transmute("res://DriftersUserDefined/pancelor-debug/Fish.tscn")
 	else:
+		world.log("the river carves a new path")
 		intend_clone(DirsOrthogonal[randi()%4])

DriftersUserDefined/pancelor-debug/River.tscn

@@ -10,7 +10,7 @@ script = ExtResource( 1 )
 guts = 5
 major_element = 2
 minor_element = 5
-evolve_skip_odds = 2000
+evolve_skip_odds = 4000
 immovable = true
 
 [node name="Sprite" type="Sprite" parent="."]

DriftersUserDefined/pancelor-debug/Sapling.gd

@@ -1,5 +1,9 @@
 extends Drifter
 
+# every so often
+func evolve():
+	tweak()
+
 func tweak():
 	world.log("new life grows")
 	intend_transmute("res://DriftersUserDefined/pancelor-debug/Tree.tscn")

docs/tutorial.md

@@ -34,7 +34,8 @@
     * `evolve_skip_odds`: higher = your drifter's `evolve` method will get called less often.
         * e.g `evolve_skip_odds=1000` means `evolve` will run on roughly 1 out of every 1000 frames
         * `evolve_skip_odds=1` means `evolve` will get called on every frame
-    * `evolve_wait_after`: number of frames of cooldown to wait before calling `evolve` again on your drifter. set it to, idk, 30 to prevent your drifter from suddenly moving a whole lot at once. up to you
+        * check out [this interactive graph](https://www.desmos.com/calculator/zuytpcsbha) to see how `evolve_skip_odds` affects how long it takes for `evolve()` to happen
+    * `evolve_wait_after`: number of frames of cooldown to wait before calling `evolve()` again on your drifter. set it to, e.g., 60 to force your drifter to `evolve()` at most once per second.
     * `immovable`: normally, objects that move into each other will swap places. however, if your drifter is marked immovable, it will not move.
 4. run the game with F5; you should see your drifter spawn! click on it to duplicate it.