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.

internal/plugin/dnd_zone_run.go

@@ -219,6 +219,15 @@ func startZoneRun(userID id.UserID, zoneID ZoneID, dndLevel int, rng *rand.Rand)
 			break
 		}
 	}
+	// Tier 6 zones are excluded from zonesForLevel by design; they're
+	// admitted here only through the post-game unlock (both T5 bosses beaten
+	// + level ≥ floor). This is the shared engine safety net — command sites
+	// give the friendly reason before reaching here.
+	if isPostgameZone(zoneID) {
+		if ok, _ := postgameUnlocked(userID, dndLevel); ok {
+			allowed = true
+		}
+	}
 	if !allowed {
 		return nil, ErrZoneTierLocked
 	}