Combat narration: format magnitude for stat_drain/debuff/ally_buff

These three stateful enemy-effect cases emitted their flavor pool via bare
pickRand, leaking the literal %d placeholder to players ("-%d damage on
every strike from here"). The magnitude is carried in CombatEvent.Damage,
exactly like the already-correct max_hp_drain / spell_fizzle cases; wrap
them in fmt.Sprintf. Add a regression test asserting no placeholder leaks.

internal/plugin/combat_narrative_drain_test.go

@@ -0,0 +1,24 @@
+package plugin
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestRenderEvent_StatefulDrainsFormatMagnitude is a regression test for the
+// leaked "%d" in stateful enemy-effect narration. stat_drain, debuff and
+// ally_buff carry their magnitude in CombatEvent.Damage and their flavor
+// pools contain a %d placeholder; renderEvent must fmt.Sprintf them rather
+// than emit the template raw (which surfaced "-%d damage" to players).
+func TestRenderEvent_StatefulDrainsFormatMagnitude(t *testing.T) {
+	for _, action := range []string{"stat_drain", "debuff", "ally_buff"} {
+		e := CombatEvent{Actor: "enemy", Action: action, Damage: 4}
+		out := renderEvent(e, "Rurina", "Shadow", CombatResult{}, newActionPicker())
+		if strings.Contains(out, "%") {
+			t.Errorf("%s: leaked format placeholder in output: %q", action, out)
+		}
+		if !strings.Contains(out, "4") {
+			t.Errorf("%s: expected magnitude 4 in output: %q", action, out)
+		}
+	}
+}