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_unplace.go

@@ -0,0 +1,77 @@
+package plugin
+
+// The Unplace branching graph (Tier 6, Phase P4).
+//
+// The unattended tear Belaxath left behind — geometry that gave up. Regions:
+// The Fray → Escher's Debt → The Stitchworks → The Seam, on the shared T6
+// skeleton. Longest walk = 46, in band.
+//
+// The Unplace's set-piece is its locks: every fork gates on a different way
+// of not-trusting-the-walls (Perception to see the true corner, INT to
+// out-think the geometry, DEX to move through a room that is also another
+// room). The secret spurs are high-LootBias impossible-geometry pockets
+// rather than named secrets — the reward is the shortcut nobody should be
+// able to take.
+
+func zoneUnplaceGraph() ZoneGraph {
+	perc := func(dc int, hint string) pgLock {
+		return pgLock{kind: LockPerception, lockData: map[string]any{"dc": dc}, hint: hint}
+	}
+	stat := func(s string, dc int, hint string) pgLock {
+		return pgLock{kind: LockStatCheck, lockData: map[string]any{"stat": s, "dc": dc}, hint: hint}
+	}
+	return buildPostgameZoneGraph(pgZoneSpec{
+		zoneID:  ZoneUnplace,
+		regions: [4]string{"unplace_the_fray", "unplace_eschers_debt", "unplace_stitchworks", "unplace_the_seam"},
+		entry:   "The Frayed Edge",
+		preamble: [12]string{
+			"The Fray", "Horizon Indoors", "The Borrowed Corner", "Room That Is Also",
+			"Angleworn Passage", "The Miss You Took", "Fraying Snare", "Silhouette Hall",
+			"The Uncounted Arms", "Escher's Approach", "Debt Landing", "The Owed Turn",
+		},
+		fork1: pgFork{
+			label:     "The First Impossibility",
+			freeLabel: [3]string{"Straight Enough Line", "Plain Corridor", "Debt Floor"},
+			lockLabel: [3]string{"Corner-Behind-You", "The Pocket That Isn't", "Folded Alcove"},
+			lock:      perc(16, "a corner you already passed is somehow ahead of you"),
+		},
+		r2gate: "Escher's Debt",
+		r2build: [6]string{
+			"Stair Up Into Down", "The Unpaid Landing", "Gallery of Wrong Angles",
+			"Nalfeshnee Concourse", "The Recursive Hall", "Stitchworks Threshold",
+		},
+		fork2: pgFork{
+			label:     "The Second Impossibility",
+			freeLabel: [3]string{"Threadbare Walk", "Loose-Weave Hall", "Stitchworks Floor"},
+			lockLabel: [3]string{"Seam-Behind-Seam", "The Unstitched Pocket", "Needled Recess"},
+			lock:      stat("INT", 17, "the geometry has a rule; find it and the shortcut is legal"),
+		},
+		r3gate: "The Stitchworks",
+		r3build: [6]string{
+			"Rows of Half-Sewn Doors", "The Needle Gallery", "Marilith Loom",
+			"Echo of Belaxath", "Seam Antechamber", "Threshold of the Tear",
+		},
+		capstone: pgCapstone{
+			label:      "The Seam Fork",
+			freeLabel:  [3]string{"Direct Seam-Walk", "The Open Tear", "Seam Floor"},
+			lockALabel: [3]string{"Sideways Step", "The Room Behind This One", "Slipped Path"},
+			lockA:      stat("DEX", 18, "step through the wall while it is also a door"),
+			lockBLabel: [3]string{"Hairline Unreality", "The Undone Pocket", "Impossible Shortcut"},
+			lockB:      perc(18, "there is a place that is not here, and it is closer than the door"),
+		},
+		merge: "The Seam",
+		approach: [5]string{
+			"The Far Half Speaking", "Sewn Into the Wound", "Needle-Rain Approach", "The Inside-Out Room", "The Last Stitch",
+		},
+		boss: "The Seamstress",
+		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(zoneUnplaceGraph())
+}