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_zones.go

@@ -0,0 +1,89 @@
+package plugin
+
+// Tier 6 "Mythic" post-game zones — shared plumbing (Phase P1).
+//
+// Post-game means earned, not leveled. zonesForLevel can't gate these
+// (playerTier = (L-1)/3+1 plus the +2 lookahead would surface a T6 zone far
+// too early — and at L18 the lookahead reaches T6 outright), so entry is
+// gated by postgameUnlocked: both Tier 5 zone bosses beaten AND level ≥ 18.
+// Locked zones still render in `!zone list` under a "Beyond the Map" divider
+// with a hint line — visible aspiration, gated entry.
+//
+// See gogobee_postgame_zones_plan.md.
+
+import (
+	"fmt"
+
+	"gogobee/internal/db"
+
+	"maunium.net/go/mautrix/id"
+)
+
+// postgameLevelFloor — the minimum character level for T6 entry. L18 lets
+// near-cap players scout and die educationally before hitting L20.
+const postgameLevelFloor = 18
+
+// postgameZones returns every registered Tier 6 zone in declared order.
+func postgameZones() []ZoneDefinition {
+	var out []ZoneDefinition
+	for _, z := range allZones() {
+		if z.Tier >= ZoneTierMythic {
+			out = append(out, z)
+		}
+	}
+	return out
+}
+
+// isPostgameZone reports whether a zone id is a Tier 6 post-game zone.
+func isPostgameZone(id ZoneID) bool {
+	z, ok := getZone(id)
+	return ok && z.Tier >= ZoneTierMythic
+}
+
+// postgameUnlocked reports whether a player may enter Tier 6 zones, and if
+// not, a one-line player-facing reason. The two gates:
+//   - both Tier 5 zone bosses (dragons_lair AND abyss_portal) beaten, and
+//   - character level ≥ postgameLevelFloor.
+//
+// The T5-clear check reuses clearedZoneIDs (boss_defeated DISTINCT query),
+// so an extraction that merely ended in 'complete' does not count.
+func postgameUnlocked(userID id.UserID, level int) (bool, string) {
+	if level < postgameLevelFloor {
+		return false, fmt.Sprintf(
+			"The way past the map's edge opens at level %d. You're level %d.",
+			postgameLevelFloor, level)
+	}
+	cleared := clearedZoneIDs(db.Get(), userID)
+	if !cleared[ZoneDragonsLair] || !cleared[ZoneAbyssPortal] {
+		return false, "The way past the map's edge stays shut until both " +
+			"Infernax and Belaxath lie dead — the dragon and the balor, " +
+			"the two ends of the known world."
+	}
+	return true, ""
+}
+
+// availableZonesFor returns the zones a player may enter for command
+// resolution: the normal level-gated set, plus the Tier 6 post-game zones
+// when postgameUnlocked. Command sites use this in place of a bare
+// zonesForLevel so an unlocked player can resolve a T6 zone id/index.
+func availableZonesFor(userID id.UserID, level int) []ZoneDefinition {
+	out := zonesForLevel(level)
+	if ok, _ := postgameUnlocked(userID, level); ok {
+		out = append(out, postgameZones()...)
+	}
+	return out
+}
+
+// postgameLockReason returns a player-facing lock reason when input names a
+// Tier 6 zone the player has NOT unlocked, otherwise "". Lets command sites
+// answer "you're not ready" instead of a generic "unknown zone" when a
+// player types a locked post-game zone by name/id.
+func postgameLockReason(input string, userID id.UserID, level int) string {
+	if _, ok := resolveZoneInput(input, postgameZones()); !ok {
+		return ""
+	}
+	if unlocked, reason := postgameUnlocked(userID, level); !unlocked {
+		return reason
+	}
+	return ""
+}