R1: split "where am I" from "how far have I walked"

Backtracking (revisit R2) breaks the assumption every zone-run caller
quietly relied on: that CurrentRoom == len(VisitedNodes)-1. Position and
progress have been the same number only because navigation was
forward-only.

Split them. CurrentRoom is now the first-entry index of CurrentNode in
VisitedNodes -- the room number the player was shown on the way in, and
the salt that enemy/trap/harvest/encounter keys hash. A revisited room
therefore resolves to the same room. RoomsTraversed is a new monotonic
step counter, persisted, backfilled from visited_nodes for in-flight rows.

The audit found only two progress-shaped reads. narrationCadence moves to
RoomsTraversed so a backtracking player draws fresh flavor rather than
replaying the entry-room lines. The other was a latent bug: zoneCmdGo
labelled the room it moved into as CurrentRoom+1, which is only correct at
the frontier; advanceZoneRunNode now returns the true path index.

VisitedNodes becomes an ordered set and RoomsCleared becomes idempotent --
both no-ops while navigation is forward-only, both load-bearing after R2.

No player-visible behavior change. Nothing to route off RoomsTraversed for
threat/SU: verified at HEAD that movement charges neither. Threat comes
from combat, supplies burn per day. The revisit plan's cost model claimed
otherwise and has been corrected -- R2's "discount" is really a net-new
cost decision.

internal/plugin/dnd_expedition_cmd.go

@@ -692,7 +692,7 @@ func (p *AdventurePlugin) autoPickStaleFork(exp *Expedition, run *DungeonRun, pf
 	if chosen == nil {
 		return false // nothing unlocked — leave it for the player / reaper
 	}
-	if err := advanceZoneRunNode(run.RunID, chosen.To); err != nil {
+	if _, err := advanceZoneRunNode(run.RunID, chosen.To); err != nil {
 		slog.Warn("expedition: auto-pick stale fork",
 			"user", run.UserID, "run", run.RunID, "err", err)
 		return false