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/postgame_zone_graph_test.go

@@ -0,0 +1,79 @@
+package plugin
+
+import "testing"
+
+// assertPostgameGraph runs the invariants every T6 zone graph must satisfy:
+// registered, longest walk in the [44,52] band, all four authored regions
+// present and matching the ExpeditionRegion registry, every fork keeps a
+// free (LockNone) exit, and all three capstone leaves reach the boss.
+func assertPostgameGraph(t *testing.T, zoneID ZoneID, build func() ZoneGraph) {
+	t.Helper()
+
+	if _, ok := zoneGraphRegistry[zoneID]; !ok {
+		t.Fatalf("%s graph not registered", zoneID)
+	}
+	g := build()
+
+	if got := graphLongestPath(g); got < 44 || got > 52 {
+		t.Errorf("%s longest path = %d, want T6 band [44,52]", zoneID, got)
+	}
+
+	// Regions: every node has a RegionID drawn from the zone's registry,
+	// and every registered region has at least one node.
+	valid := map[string]bool{}
+	for _, r := range regionsByZone[zoneID] {
+		valid[r.ID] = true
+	}
+	if len(valid) != 4 {
+		t.Fatalf("%s: expected 4 registered regions, got %d", zoneID, len(valid))
+	}
+	seen := map[string]int{}
+	for id, n := range g.Nodes {
+		if n.RegionID == "" {
+			t.Errorf("%s node %s has empty RegionID", zoneID, id)
+			continue
+		}
+		if !valid[n.RegionID] {
+			t.Errorf("%s node %s RegionID %q not in region registry", zoneID, id, n.RegionID)
+		}
+		seen[n.RegionID]++
+	}
+	for r := range valid {
+		if seen[r] == 0 {
+			t.Errorf("%s region %q has no nodes", zoneID, r)
+		}
+	}
+
+	// Every fork offers at least one LockNone exit (local mirror of the
+	// global TestZoneGraphs_NoSoftLockedFork, kept here so a broken spec
+	// fails in its own zone's test too).
+	for id, n := range g.Nodes {
+		if n.Kind != NodeKindFork {
+			continue
+		}
+		free := false
+		for _, e := range g.outgoingEdges(id) {
+			if e.Lock == LockNone || e.Lock == "" {
+				free = true
+			}
+		}
+		if !free {
+			t.Errorf("%s fork %s: no free exit (soft-lock)", zoneID, id)
+		}
+	}
+
+	// All three capstone leaves reach the boss.
+	for _, leaf := range []string{
+		string(zoneID) + ".cap13",
+		string(zoneID) + ".cap23",
+		string(zoneID) + ".cap33",
+	} {
+		if _, ok := g.Nodes[leaf]; !ok {
+			t.Errorf("%s missing capstone leaf %s", zoneID, leaf)
+			continue
+		}
+		if !reachable(g, leaf, g.Boss) {
+			t.Errorf("%s capstone leaf %s unreachable to boss", zoneID, leaf)
+		}
+	}
+}