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/zone_graph_first_hoard_test.go

@@ -0,0 +1,32 @@
+package plugin
+
+import "testing"
+
+func TestFirstHoardGraph_Invariants(t *testing.T) {
+	assertPostgameGraph(t, ZoneFirstHoard, zoneFirstHoardGraph)
+}
+
+// TestFirstHoardGraph_GameMaxLootBias verifies the plan's requirement that
+// the First Hoard carries the game-max LootBias route (2.5–3.0), which the
+// boss's Greed Tax self-balances. The deepest gilded vein hits the 3.0
+// ceiling and at least three nodes sit at or above 2.5.
+func TestFirstHoardGraph_GameMaxLootBias(t *testing.T) {
+	g := zoneFirstHoardGraph()
+
+	var max float64
+	rich := 0
+	for _, n := range g.Nodes {
+		if n.Content.LootBias > max {
+			max = n.Content.LootBias
+		}
+		if n.Content.LootBias >= 2.5 {
+			rich++
+		}
+	}
+	if max < 3.0 {
+		t.Errorf("max LootBias = %v, want >= 3.0 (game-max gilded route)", max)
+	}
+	if rich < 3 {
+		t.Errorf("nodes with LootBias >= 2.5 = %d, want >= 3", rich)
+	}
+}