Restore expedition completion on zone clear (lost in merge)

finalizeExpeditionOnZoneClear and its wiring into the run-complete seam
were introduced in 73b7809 but that commit never made it into the
current history — it's not an ancestor of HEAD, so the function, its
call site in advanceOnceWithOpts, and its tests were all absent.

Symptom: clearing a zone (boss down, no outgoing edges) set the run's
boss_defeated flag but never flipped the wrapping expedition to
'complete' or retired the run. The expedition sat 'active' pointing at a
boss_defeated run, so getActiveZoneRun (filters boss_defeated=0) returned
nil and the next '!expedition run' errored with 'No active zone run'. The
ambient ticker also kept DMing about a finished dungeon.

Re-apply the bridge verbatim against current HEAD (deps verified present:
IsMultiRegionZone / CurrentRegion / MarkRegionBossDefeated /
completeExpedition / retireAllRegionRuns / AwardCompletionMilestones) and
restore the test.

internal/plugin/dnd_zone_cmd.go

@@ -604,6 +604,16 @@ func (p *AdventurePlugin) advanceOnceWithOpts(ctx MessageContext, compact bool)
 				b.WriteString("• " + id + "\n")
 			}
 		}
+		// Success-path expedition close-out: flip the wrapping expedition to
+		// 'complete' (when this clear finishes the whole zone) and surface any
+		// completion milestones. No-op for standalone runs / mid-zone region
+		// clears.
+		if lines := p.finalizeExpeditionOnZoneClear(ctx.Sender, run.RunID); len(lines) > 0 {
+			b.WriteString("\n")
+			for _, line := range lines {
+				b.WriteString(line)
+			}
+		}
 		return advanceResult{
 			preStream: preStream,
 			intro:     intro,