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_last_meridian.go

@@ -0,0 +1,75 @@
+package plugin
+
+// The Last Meridian branching graph (Tier 6, Phase P4).
+//
+// A necropolis-observatory being decommissioned hour by hour. Regions: The
+// Dusk Colonnade → Gallery of Spent Hours → The Escapement → One Minute To
+// Midnight, on the shared T6 skeleton. Longest walk = 46, in band.
+//
+// A time zone: the forks gate on reading the clockwork's rhythm (INT for the
+// escapement's rule, DEX to slip a stolen turn, WIS to keep your own hour
+// when the Hour Thief reaches for it). The locked spurs are pockets of
+// unspent time — high LootBias hours the Custodian hasn't dismantled yet.
+
+func zoneLastMeridianGraph() ZoneGraph {
+	stat := func(s string, dc int, hint string) pgLock {
+		return pgLock{kind: LockStatCheck, lockData: map[string]any{"stat": s, "dc": dc}, hint: hint}
+	}
+	perc := func(dc int, hint string) pgLock {
+		return pgLock{kind: LockPerception, lockData: map[string]any{"dc": dc}, hint: hint}
+	}
+	return buildPostgameZoneGraph(pgZoneSpec{
+		zoneID:  ZoneLastMeridian,
+		regions: [4]string{"meridian_dusk_colonnade", "meridian_gallery_spent_hours", "meridian_escapement", "meridian_one_minute"},
+		entry:   "The Dusk Gate",
+		preamble: [12]string{
+			"The Dusk Colonnade", "Hall of Winding Nothing", "The Hour Thief's Trail", "Candle-Shortened Walk",
+			"Horologist's Ghost-Aisle", "The Stolen Present", "Pickpocket's Snare", "Colonnade of Spent Light",
+			"The Gallery Approach", "Gallery Mouth", "Hall of Spent Hours", "First Struck Bell",
+		},
+		fork1: pgFork{
+			label:     "The First Hour",
+			freeLabel: [3]string{"On-Time Passage", "The Kept Hour", "Gallery Floor"},
+			lockLabel: [3]string{"Guarded Recess", "The Unspent Hour-Pocket", "Ticking Alcove"},
+			lock:      stat("WIS", 16, "keep your own hour when the Thief reaches for it and a door stays open"),
+		},
+		r2gate: "Gallery of Spent Hours",
+		r2build: [6]string{
+			"Rows of Stopped Clocks", "The Twin Wardens' Door", "Changing of the Guard",
+			"Shift-Schedule Hall", "The Nothing Behind", "Escapement Threshold",
+		},
+		fork2: pgFork{
+			label:     "The Second Hour",
+			freeLabel: [3]string{"Steady Escapement Walk", "The Even-Round Hall", "Escapement Floor"},
+			lockLabel: [3]string{"Gear-Seam", "The Unwound Vault", "Pendulum Niche"},
+			lock:      stat("INT", 17, "the pendulum only swings on even rounds — read the rhythm and slip past"),
+		},
+		r3gate: "The Escapement",
+		r3build: [6]string{
+			"The Original Pendulum", "Gallery of Its Legs", "Verdigris Colossus Walk",
+			"Orrery-Ring Landing", "Midnight Antechamber", "Threshold of the Last Hour",
+		},
+		capstone: pgCapstone{
+			label:      "The Midnight Fork",
+			freeLabel:  [3]string{"Direct Midnight-Walk", "The Open Escapement", "Midnight Floor"},
+			lockALabel: [3]string{"Borrowed Second", "The Slipped Turn", "Stolen-Time Path"},
+			lockA:      stat("DEX", 18, "steal a second from the clock and step through before it notices"),
+			lockBLabel: [3]string{"Unwound Seam", "The Hour Never Struck", "Unspent Shortcut"},
+			lockB:      perc(18, "one hour on the great face was never dismantled — it is still open"),
+		},
+		merge: "One Minute To Midnight",
+		approach: [5]string{
+			"The Apologizing Golem", "Orrery of the Last Hour", "Chime Approach", "The Rewinding Second", "The Clock Stops",
+		},
+		boss: "The Custodian of the Last Hour",
+		overrides: map[string]pgNodeOverride{
+			"f1b2":  {kind: NodeKindSecret, bias: 2.25},
+			"f2b2":  {kind: NodeKindSecret, bias: 2.25},
+			"cap32": {kind: NodeKindSecret, bias: 2.5},
+		},
+	})
+}
+
+func init() {
+	registerZoneGraph(zoneLastMeridianGraph())
+}