Combat: wire turn-based elite/boss fights to the command surface

Routes Elite/Boss rooms off the auto-resolve SimulateCombat path and onto
the persisted turn-based engine. !zone advance now stops at an Elite/Boss
doorway; the player engages with !fight, then resolves one full round per
!attack / !flee. A won CombatSession is the record that the room's combat
is done, so a fresh !zone advance clears the room and advances the graph.

- buildZoneCombatants: shared player/enemy Combatant builder extracted from
  runZoneCombat; combatantsForSession rebuilds the pair from a session row.
- runCombatRound loops the phase state machine through a whole round;
  finishCombatSession runs HP/XP/loot/kill/threat/mood close-out.
- getCombatSessionForEncounter lets the room resolver tell "already won"
  apart from "not yet fought".
- !zone advance/enter/go blocked while a session is active.
- resolveBossRoom deleted (dead after the reroute).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/dnd_zone_cmd_graph.go

@@ -161,6 +161,9 @@ func (p *AdventurePlugin) zoneCmdGo(ctx MessageContext, rest string) error {
 	if run == nil {
 		return p.SendDM(ctx.Sender, "No active zone run. Use `!zone enter <id>`.")
 	}
+	if cs, _ := getActiveCombatSession(ctx.Sender); cs != nil {
+		return p.SendDM(ctx.Sender, "⚔️ Finish your fight first — `!attack` or `!flee`.")
+	}
 	pf, derr := decodePendingFork(run.NodeChoices)
 	if derr != nil {
 		return p.SendDM(ctx.Sender, "Couldn't decode pending fork: "+derr.Error())