robbie: pay for the haul, not just for showing up

The gift was one consumable every 10th visit, flat. A visit is a 40%
daily roll, so that works out to one item per ~25 real days — and it paid
exactly the same for a stockpile of sixty items as it did for one rock.
The player controls volume, not visit count, so volume is what the new
track pays on: one consumable per 15 items carried off, capped at 3,
stacking with the existing loyalty gift.

Also stop Robbie stealing thieves' tools. He skips keys already, for
exactly this reason — a key is bought to open something later, and a
bandit who pockets it between the purchase and the door has taken the
thing the player paid to still have. Tools are the same shape of promise.

internal/plugin/adventure_robbie_gift_test.go

@@ -0,0 +1,47 @@
+package plugin
+
+import "testing"
+
+// TestRobbieGiftCount pins the two tracks: the every-10th-visit loyalty gift
+// and the volume track that pays for a big haul, capped so one monster
+// stockpile can't mint an unbounded pile of consumables.
+func TestRobbieGiftCount(t *testing.T) {
+	cases := []struct {
+		name          string
+		visits, taken int
+		want          int
+	}{
+		{"small haul, off-loyalty visit", 7, 3, 0},
+		{"loyalty visit only", 10, 3, 1},
+		{"haul only", 7, 15, 1},
+		{"haul and loyalty stack", 20, 15, 2},
+		{"haul scales", 7, 45, 3},
+		{"haul capped", 7, 500, robbieMaxHaulGifts},
+		{"cap plus loyalty", 30, 500, robbieMaxHaulGifts + 1},
+		{"one under the haul threshold", 7, robbieHaulPerGift - 1, 0},
+		{"zeroth visit is not a loyalty visit", 0, 0, 0},
+	}
+	for _, c := range cases {
+		if got := robbieGiftCount(c.visits, c.taken); got != c.want {
+			t.Errorf("%s: robbieGiftCount(%d, %d) = %d, want %d",
+				c.name, c.visits, c.taken, got, c.want)
+		}
+	}
+}
+
+func TestJoinAnd(t *testing.T) {
+	cases := []struct {
+		in   []string
+		want string
+	}{
+		{nil, ""},
+		{[]string{"a"}, "a"},
+		{[]string{"a", "b"}, "a and b"},
+		{[]string{"a", "b", "c"}, "a, b and c"},
+	}
+	for _, c := range cases {
+		if got := joinAnd(c.in); got != c.want {
+			t.Errorf("joinAnd(%v) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}