adventure: route round-end concentration kill through enemyDown (P8)

The T6 Valdris phylactery rebirth (and the pre-existing survive_at_1
one-shot) live in enemyDown, on the premise that it is the single death
chokepoint. It isn't: the round-end concentration pulse ended the fight
on a raw enemyHP<=0 read, so a cleric's Spirit Guardians landing the
lethal blow robbed a revive-armed boss of its rebirth — exactly the
cleric-party arm P8 is tuned around. Route that win-check through
enemyDown so the boss gets its chance to stand back up. Regression test
covers both the armed (revives) and charge-less (still wins) paths.

Claude-Session: https://claude.ai/code/session_0156WqjgsbmSY2U8eQ3Kkb1s

internal/plugin/combat_turn_engine.go

@@ -842,7 +842,12 @@ func (te *turnEngine) stepRoundEnd() {
 			Round: st.round, Phase: CombatPhaseRoundEnd, Actor: "player", Action: "concentration_tick",
 			Damage: st.concentrationDmg, PlayerHP: st.playerHP, EnemyHP: st.enemyHP, Seat: i,
 		})
-		if st.enemyHP <= 0 {
+		// Route the kill through enemyDown, not a raw HP read: a boss that cheats
+		// death (survive_at_1) or holds a phylactery rebirth (T6 Valdris) must get
+		// that chance even when the lethal blow is a lingering concentration pulse.
+		// enemyDown restores its HP and returns false, so the next seat's pulse (or
+		// the following round) resolves against the revived pool.
+		if enemyDown(st, CombatPhaseRoundEnd) {
 			te.finish(CombatStatusWon)
 			return
 		}