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/expedition_party_cmd.go

@@ -137,7 +137,16 @@ func (p *AdventurePlugin) expeditionCmdAccept(ctx MessageContext, c *DnDCharacte
 	}
 
 	zone := zoneOrFallback(exp.ZoneID)
-	if !zoneOpenToLevel(exp.ZoneID, c.Level) {
+	// Tier 6 zones are excluded from zonesForLevel by design, so zoneOpenToLevel
+	// would refuse every seat; they carry their own post-game unlock instead.
+	// Non-postgame zones keep the plain level gate.
+	if isPostgameZone(exp.ZoneID) {
+		// A party can't smuggle an ungated member into a Tier 6 zone: every seat
+		// must have cleared the post-game unlock on their own.
+		if ok, reason := postgameUnlocked(ctx.Sender, c.Level); !ok {
+			return p.SendDM(ctx.Sender, reason)
+		}
+	} else if !zoneOpenToLevel(exp.ZoneID, c.Level) {
 		return p.SendDM(ctx.Sender, fmt.Sprintf(
 			"**%s** is beyond you for now — come back at a higher level.", zone.Display))
 	}