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_ossuary_ascendant_test.go

@@ -0,0 +1,61 @@
+package plugin
+
+import "testing"
+
+func TestOssuaryAscendantGraph_Invariants(t *testing.T) {
+	assertPostgameGraph(t, ZoneOssuaryAscendant, zoneOssuaryAscendantGraph)
+}
+
+// TestOssuaryAscendantGraph_ThreeVerses verifies the signature set-piece:
+// three NodeKindSecret Verses, each reached through a LockPerception spur
+// edge (the gate rides the fork→threshold edge; the edge into the Verse
+// itself is unlocked).
+func TestOssuaryAscendantGraph_ThreeVerses(t *testing.T) {
+	g := zoneOssuaryAscendantGraph()
+
+	verses := []string{"ossuary_ascendant.f1b2", "ossuary_ascendant.f2b2", "ossuary_ascendant.cap32"}
+	for _, v := range verses {
+		n, ok := g.Nodes[v]
+		if !ok {
+			t.Errorf("missing Verse node %s", v)
+			continue
+		}
+		if n.Kind != NodeKindSecret {
+			t.Errorf("Verse %s kind = %q, want secret", v, n.Kind)
+		}
+	}
+	if got := countSecretNodes(g); got != 3 {
+		t.Errorf("secret nodes = %d, want 3 (the Phylactery Verses)", got)
+	}
+
+	// Each Verse's spur is entered through a Perception gate.
+	percThresholds := map[string]bool{
+		"ossuary_ascendant.f1b1": false,
+		"ossuary_ascendant.f2b1": false,
+		"ossuary_ascendant.cap31": false,
+	}
+	for _, outs := range g.Edges {
+		for _, e := range outs {
+			if e.Lock == LockPerception {
+				if _, ok := percThresholds[e.To]; ok {
+					percThresholds[e.To] = true
+				}
+			}
+		}
+	}
+	for th, gated := range percThresholds {
+		if !gated {
+			t.Errorf("Verse threshold %s is not behind a LockPerception edge", th)
+		}
+	}
+}
+
+func countSecretNodes(g ZoneGraph) int {
+	n := 0
+	for _, node := range g.Nodes {
+		if node.Kind == NodeKindSecret {
+			n++
+		}
+	}
+	return n
+}