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/dnd_zone_test.go

@@ -128,10 +128,10 @@ func TestZoneRegistry_LootDropChances(t *testing.T) {
 func TestZoneRegistry_RoomCountSane(t *testing.T) {
 	// Long-expedition plan §2 widens the bands per tier: T1 12–14 up to
 	// T5 ~36–44. The guard floor stays at 5 (no zone should ever drop
-	// below that) and the ceiling tracks the T5 target.
+	// below that) and the ceiling tracks the T6 post-game target of 44–52.
 	for _, z := range allZones() {
-		if z.MinRooms < 5 || z.MaxRooms > 44 || z.MinRooms > z.MaxRooms {
-			t.Errorf("zone %s rooms %d-%d outside design (5-44, min<=max)",
+		if z.MinRooms < 5 || z.MaxRooms > 52 || z.MinRooms > z.MaxRooms {
+			t.Errorf("zone %s rooms %d-%d outside design (5-52, min<=max)",
 				z.ID, z.MinRooms, z.MaxRooms)
 		}
 	}
@@ -162,6 +162,7 @@ func TestZoneRegistry_LevelRangeMatchesTier(t *testing.T) {
 		ZoneTierJourneyman: {5, 8},
 		ZoneTierVeteran:    {8, 12},
 		ZoneTierLegendary:  {12, 20},
+		ZoneTierMythic:     {18, 20}, // post-game: gated by unlock, not level band
 	}
 	for _, z := range allZones() {
 		want, ok := expected[z.Tier]