adventure: Tier 6 postgame "Mythic" dungeons (P1–P7 calibration)

Five post-game dungeons above the T5 ceiling, gated on both T5 bosses beaten
+ level 18. Opt-in endgame: deadly solo, clearable by a party with Pete + pets.

- P1 gating: postgameUnlocked (T5 clears + level floor), zonesForLevel excludes
  T6 unconditionally; wired into startZoneRun, !zone/!expedition, party accept,
  boredom picker, and the list dividers.
- P2 bestiary: 15 elites + 5 signature bosses (Layer-1 stat blocks).
- P3 zone defs + 4-region registries; ZoneLootEntry.BossOnly.
- P4 five zone graphs on a shared builder (44–52 rooms, no soft-lock; Ossuary
  secret Verse nodes).
- P5 loot: BossOnly enforced; signature items are real registry magic items;
  five Thom pity recipes off the per-zone crafting anchors.
- P6 narration/flavor (5 files), T6 achievements, Pete stays zone-parametric.
- P7 (in progress): sim can now reach gated T6 (SimRunner.SeedPostgameUnlock +
  IsPostgameZone). First calibration pass on millenia — hardened ossuary +
  drowned_star, softened first_hoard + unplace; last_meridian in band.

Fix: party members were refused from every T6 zone because expeditionCmdAccept
ran the level gate (which excludes T6) before the postgame check — the intended
party endgame was unreachable. Route T6 through postgameUnlocked. Regression
tests added.

cmd/expedition-sim/main.go

@@ -342,6 +342,17 @@ func runOne(dataDir string, uid id.UserID, class plugin.DnDClass, level int, zon
 		runner.Euro.Credit(muid, bank, "expedition-sim bankroll")
 		members = append(members, muid)
 	}
+	// T6 post-game zones are gated on both T5 bosses beaten (+ level floor).
+	// The synthetic party has no expedition history, so seed the two clears
+	// for every party member — the per-member unlock check in !expedition
+	// accept would otherwise refuse the whole party at the zone's edge.
+	if plugin.IsPostgameZone(zone) {
+		for _, m := range append([]id.UserID{uid}, members...) {
+			if err := runner.SeedPostgameUnlock(m); err != nil {
+				return nil, fmt.Errorf("seed postgame unlock: %w", err)
+			}
+		}
+	}
 	runner.Companion = companion
 	return runner.RunPartyExpedition(uid, members, zone, cap, days)
 }