Branching zones G9b: stop persisting current_room / room_seq_json

The two legacy columns are no longer needed at runtime. CurrentRoom is
derived from len(VisitedNodes)-1 on read (lockstep with what the
dual-write era stored), and RoomSeq is a transient in-memory field
populated only when a fresh run is started.

- INSERT, SELECT, and UPDATE statements drop the two columns. The
  schema still carries them — they retire in G9c.
- scanZoneRun derives CurrentRoom and falls back to a linear-derived
  CurrentNode only if a row predates the G4 dual-write deploy.
- markRoomCleared rewritten to walk the registered graph (first
  outgoing edge / dead-end completion). Tests that use it as an
  end-to-end run driver continue to pass.
- advanceZoneRunNode drops its current_room dual-write.
- adventure_activity_unified reads visited_nodes for the daily
  "X/N rooms" progress fragment.
- Camp boss-room test pivots to setting current_node directly.

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

internal/plugin/dnd_expedition_camp_test.go

@@ -267,24 +267,27 @@ func TestCampLocationCheck_BossRoomBlocks(t *testing.T) {
 	defer cleanupZoneRuns(uid)
 	defer cleanupExpeditions(uid)
 
-	// Pre-create a zone run, then advance current_room to the boss room.
+	// Pre-create a zone run, then warp current_node to the graph's boss.
 	run, err := startZoneRun(uid, ZoneGoblinWarrens, 1, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
-	// Walk to the boss room directly (last index).
-	bossIdx := -1
-	for i, rt := range run.RoomSeq {
-		if rt == RoomBoss {
-			bossIdx = i
+	g, ok := loadZoneGraph(run.ZoneID)
+	if !ok {
+		t.Fatalf("no graph for zone %q", run.ZoneID)
+	}
+	bossNodeID := ""
+	for nid, n := range g.Nodes {
+		if n.IsBoss {
+			bossNodeID = nid
+			break
 		}
 	}
-	if bossIdx < 0 {
-		t.Fatal("no boss room in seq")
+	if bossNodeID == "" {
+		t.Fatal("no boss node in graph")
 	}
 	exp := &Expedition{RunID: run.RunID}
-	// Force current_room to bossIdx.
-	if _, err := db.Get().Exec(`UPDATE dnd_zone_run SET current_room = ? WHERE run_id = ?`, bossIdx, run.RunID); err != nil {
+	if _, err := db.Get().Exec(`UPDATE dnd_zone_run SET current_node = ? WHERE run_id = ?`, bossNodeID, run.RunID); err != nil {
 		t.Fatal(err)
 	}
 	cleared, problem := campLocationCheck(exp)