Pets: roll arrival on expedition emergence, not the 08:00 morning DM

The pet-arrival roll only ran in sendMorningDMs (the 08:00 overworld
morning DM), which is skipped for anyone underground. Expedition players
are almost never in the overworld at 08:00, so the roll never reached
them — the encounter never fired despite the conditions being met.

Move the roll onto the emergence seam via a shared helper
maybeRollPetArrivalOnEmerge, called when a player surfaces alive
(voluntary extract, abandon, survived forced extraction) and on respawn
for underground deaths. Remove the now-dead morning-DM arrival roll.
Story-wise: while the player was out, an animal wandered into the empty
house looking for food.

The 25% morning pet event still rolls only in the overworld DM and has
the same reachability gap; left for a follow-up.

internal/plugin/dnd_expedition_cmd.go

@@ -486,9 +486,14 @@ func (p *AdventurePlugin) expeditionCmdAbandon(ctx MessageContext) error {
 	}
 	_ = retireAllRegionRuns(exp)
 	_ = appendExpeditionLog(exp.ID, exp.CurrentDay, "narrative", "expedition abandoned", "")
-	return p.SendDM(ctx.Sender, fmt.Sprintf(
+	if err := p.SendDM(ctx.Sender, fmt.Sprintf(
 		"Expedition in **%s** abandoned on Day %d. Supplies are forfeit. The dungeon remembers.",
-		zone.Display, exp.CurrentDay))
+		zone.Display, exp.CurrentDay)); err != nil {
+		return err
+	}
+	// Emergence seam: see maybeRollPetArrivalOnEmerge.
+	p.maybeRollPetArrivalOnEmerge(ctx.Sender)
+	return nil
 }
 
 // helper: ensure we don't shadow id.UserID import in test harness.