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_boredom.go

@@ -375,6 +375,12 @@ func isBoredomDriven(userID id.UserID, now time.Time) bool {
 func pickBoredomZone(uid id.UserID, level int) (ZoneDefinition, bool) {
 	var inBand, nonRaid []ZoneDefinition
 	for _, z := range allZones() {
+		// Tier 6 post-game zones are opt-in endgame — a bored character never
+		// wanders into one on autopilot, even once unlocked. This picker walks
+		// allZones() directly (not zonesForLevel), so exclude T6 explicitly.
+		if z.Tier >= ZoneTierMythic {
+			continue
+		}
 		if level < z.LevelMin || level > z.LevelMax {
 			continue
 		}