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_balance_test.go

@@ -123,6 +123,11 @@ var phase1TierCenterline = map[ZoneTier]int{
 	ZoneTierJourneyman: 9,
 	ZoneTierVeteran:    13,
 	ZoneTierLegendary:  17,
+	// Tier 6 post-game runs at the cap. These in-process matrices tune the
+	// T1–T5 global levers; T6 rows are simmed and logged for reference but
+	// carry no band assertion here — T6 balance is owned by the remote P7
+	// sweep (n=750, control arm) per gogobee_postgame_zones_plan.md §4.
+	ZoneTierMythic: 20,
 }
 
 // TestExpeditionBalance_Phase1_FullMatrix is the Phase 1 baseline-