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/achievements.go

@@ -1205,6 +1205,11 @@ func (p *AchievementsPlugin) buildAchievements() []achievementDef {
 			Emoji: "🗺️",
 			Check: func(d *sql.DB, u id.UserID) bool { return clearedAnyZoneOfTier(d, u, 5) },
 		},
+		{
+			ID: "expedition_clear_t6", Name: "Off the Edge of the Map", Description: "Cleared a Mythic zone. The map never went this far. Neither should you have.",
+			Emoji: "🗺️",
+			Check: func(d *sql.DB, u id.UserID) bool { return clearedAnyZoneOfTier(d, u, 6) },
+		},
 		{
 			ID: "expedition_master_t1", Name: "Warren Cartography", Description: "Cleared every Tier 1 zone. Thorough.",
 			Emoji: "🧭",
@@ -1230,6 +1235,11 @@ func (p *AchievementsPlugin) buildAchievements() []achievementDef {
 			Emoji: "🧭",
 			Check: func(d *sql.DB, u id.UserID) bool { return clearedEveryZoneOfTier(d, u, 5) },
 		},
+		{
+			ID: "expedition_master_t6", Name: "There Was No Sixth Tier", Description: "Cleared every Mythic zone. Officially, none of these existed.",
+			Emoji: "🧭",
+			Check: func(d *sql.DB, u id.UserID) bool { return clearedEveryZoneOfTier(d, u, 6) },
+		},
 		{
 			ID: "expedition_quiet_clear", Name: "Nobody Saw Anything", Description: "Cleared a zone without threat ever passing 50. You were never here.",
 			Emoji: "🌑",