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/magic_items_gameplay.go

@@ -184,9 +184,10 @@ type magicItemEffect struct {
 }
 
 // magicItemEffectOverlay — hand-authored per-item effects that win over the
-// codified formula. Empty for now; corrections land here rather than being
-// folded into the formula, mirroring magicItemOverlay in magic_items.go.
-var magicItemEffectOverlay = map[string]magicItemEffect{}
+// codified formula. Corrections land here rather than being folded into the
+// formula, mirroring magicItemOverlay in magic_items.go. Today it carries the
+// T6 signature items' bespoke combat numbers (postgame_magic_items.go).
+var magicItemEffectOverlay = postgameSignatureEffects
 
 // rarityPowerScalar is the codified power axis: rarer item, bigger delta.
 func rarityPowerScalar(r DnDRarity) float64 {