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_drowned_star.go

@@ -0,0 +1,75 @@
+package plugin
+
+// The Drowned Star branching graph (Tier 6, Phase P4).
+//
+// The abyssal trench where Seraphel rode her star down. Regions: The Long
+// Sink → Pilgrim Trench → The Radiant Wreck → The Heart Chapel, on the
+// shared T6 skeleton. Longest walk = 46, in band.
+//
+// A descent zone: the forks gate on holding your nerve in the dark (CON
+// against the pressure, WIS against the false lights, CHA to walk the
+// pilgrim route unmolested). The locked spurs are radiant pockets of the
+// wreck — high LootBias, lit by the dying star.
+
+func zoneDrownedStarGraph() 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:  ZoneDrownedStar,
+		regions: [4]string{"star_long_sink", "star_pilgrim_trench", "star_radiant_wreck", "star_heart_chapel"},
+		entry:   "The Waterline",
+		preamble: [12]string{
+			"The Long Sink", "Descending Dark", "Pressure Gate", "The Static Shoal",
+			"Choir of Static", "Sunless Landing", "Undertow Snare", "The Glow Below",
+			"Pilgrim's Approach", "Trench Mouth", "The Lit Road", "First Lantern",
+		},
+		fork1: pgFork{
+			label:     "The First Descent",
+			freeLabel: [3]string{"Marked Pilgrim Road", "Lantern-Lit Walk", "Trench Floor"},
+			lockLabel: [3]string{"Off-Route Pocket", "The Radiant Wreck-Pocket", "Sunken Alcove"},
+			lock:      stat("CON", 16, "the pressure here would fold a lesser diver — endure it and the wreck opens"),
+		},
+		r2gate: "Pilgrim Trench",
+		r2build: [6]string{
+			"Trench of the Followed Light", "The Angler-Knight's Walk", "False-Light Gallery",
+			"Warden's Lure", "The Unseen-Again Hall", "Wreck Threshold",
+		},
+		fork2: pgFork{
+			label:     "The Second Descent",
+			freeLabel: [3]string{"Steady Pilgrim Line", "Trusted Dark", "Wreck Floor"},
+			lockLabel: [3]string{"Halo-Fragment Recess", "The Star-Lit Vault", "Radiant Niche"},
+			lock:      stat("WIS", 17, "one lantern is not a lure — know which and it leads true"),
+		},
+		r3gate: "The Radiant Wreck",
+		r3build: [6]string{
+			"The Star's Broken Hull", "Gallery of Ten Thousand Years", "Leviathanling Roost",
+			"The Glowing Silhouette", "Chapel Antechamber", "Threshold of the Heart",
+		},
+		capstone: pgCapstone{
+			label:      "The Chapel Fork",
+			freeLabel:  [3]string{"Direct Chapel-Walk", "The Open Reredos", "Chapel Floor"},
+			lockALabel: [3]string{"Pilgrim's Grace", "The Unmolested Route", "Sanctified Path"},
+			lockA:      stat("CHA", 18, "walk the pilgrim route as a pilgrim and it lets you pass"),
+			lockBLabel: [3]string{"Halo-Seam", "The Heart's Own Light", "Radiant Shortcut"},
+			lockB:      perc(18, "a crack in the radiance where the star still leaks its oldest light"),
+		},
+		merge: "The Heart Chapel",
+		approach: [5]string{
+			"The Shielded Star", "Seraphel's Vigil", "Sanctified Undertow Walk", "The Grieving Light", "The Last Radiance",
+		},
+		boss: "Seraphel, the Light That Sank",
+		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(zoneDrownedStarGraph())
+}