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/adventure_activity_unified.go

@@ -1,6 +1,7 @@
 package plugin
 
 import (
+	"encoding/json"
 	"fmt"
 	"time"
 
@@ -78,9 +79,10 @@ func loadAdvDailyActivity(date string) (map[id.UserID][]AdvDailyActivity, error)
 	}
 	rows.Close()
 
-	// 2. dnd_zone_run — rows touched today.
+	// 2. dnd_zone_run — rows touched today. Progress count is derived
+	// from len(visited_nodes) — current_room retired in G9.
 	rows, err = d.Query(`
-		SELECT user_id, zone_id, current_room, total_rooms, abandoned,
+		SELECT user_id, zone_id, visited_nodes, total_rooms, abandoned,
 		       boss_defeated, completed_at, last_action_at
 		FROM dnd_zone_run
 		WHERE last_action_at >= ? AND last_action_at < DATE(?, '+1 day')
@@ -91,16 +93,23 @@ func loadAdvDailyActivity(date string) (map[id.UserID][]AdvDailyActivity, error)
 	for rows.Next() {
 		var (
 			uid, zoneID                        string
-			currentRoom, totalRooms            int
+			visitedJSON                        string
+			totalRooms                         int
 			abandoned, bossDefeated            int
 			completedAt                        *time.Time
 			lastAction                         time.Time
 		)
-		if err := rows.Scan(&uid, &zoneID, &currentRoom, &totalRooms,
+		if err := rows.Scan(&uid, &zoneID, &visitedJSON, &totalRooms,
 			&abandoned, &bossDefeated, &completedAt, &lastAction); err != nil {
 			rows.Close()
 			return nil, fmt.Errorf("zone run scan: %w", err)
 		}
+		var visited []string
+		_ = json.Unmarshal([]byte(visitedJSON), &visited)
+		currentRoom := len(visited) - 1
+		if currentRoom < 0 {
+			currentRoom = 0
+		}
 		userID := id.UserID(uid)
 		zoneDef := zoneOrFallback(ZoneID(zoneID))