Branching zones G8e: Underforge pre-boss gauntlet (late triple fork)

T3 zone, fifth authored graph. Shape: five-node linear preamble
(sealed_gate → forge_descent → cooling_river → magma_chamber elite)
followed by a single 3-way antechamber fork before the boss.

Bends the plan §G8 "two forks for T2+" guideline to a single late
fork-with-three-options because the gauntlet shape is the design
intent — Kharak Dûn is a one-way descent. Triple-option antechamber
(direct / DEX-checked catwalks / Perception-found forge_vault secret)
preserves agency at the commitment moment.

Map renders as a long horizontal stem with a 3-leaf fan at the right
edge — visually distinct from any earlier zone.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/zone_graph_underforge_test.go

@@ -0,0 +1,50 @@
+package plugin
+
+import "testing"
+
+func TestUnderforgeGraph_Registered(t *testing.T) {
+	g, ok := zoneGraphRegistry[ZoneUnderforge]
+	if !ok {
+		t.Fatal("zoneUnderforgeGraph not registered")
+	}
+	if len(g.Nodes) != 10 {
+		t.Errorf("nodes = %d, want 10", len(g.Nodes))
+	}
+}
+
+// TestUnderforgeGraph_LinearPreamble locks in the gauntlet shape:
+// the first five nodes after entry must each have exactly one outgoing
+// edge (linear chain). If a future edit splits the preamble, this test
+// catches it — that change should re-author the shape comment too.
+func TestUnderforgeGraph_LinearPreamble(t *testing.T) {
+	g := zoneUnderforgeGraph()
+	for _, id := range []string{
+		"underforge.entry",
+		"underforge.sealed_gate",
+		"underforge.forge_descent",
+		"underforge.cooling_river",
+		"underforge.magma_chamber",
+	} {
+		outs := g.outgoingEdges(id)
+		if len(outs) != 1 {
+			t.Errorf("preamble node %s outgoing = %d, want 1 (gauntlet)", id, len(outs))
+		}
+	}
+}
+
+func TestUnderforgeGraph_AntechamberThreeWay(t *testing.T) {
+	g := zoneUnderforgeGraph()
+	outs := g.outgoingEdges("underforge.antechamber")
+	if len(outs) != 3 {
+		t.Fatalf("antechamber outgoing = %d, want 3", len(outs))
+	}
+	for _, leaf := range []string{
+		"underforge.direct_assault",
+		"underforge.catwalks",
+		"underforge.forge_vault",
+	} {
+		if !reachable(g, leaf, "underforge.boss") {
+			t.Errorf("%s unreachable to boss", leaf)
+		}
+	}
+}