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/db/db.go

@@ -345,6 +345,13 @@ func runMigrations(d *sql.DB) error {
 		// needs no bootstrap backfill.
 		`ALTER TABLE adventure_inventory ADD COLUMN temper INTEGER NOT NULL DEFAULT 0`,
 		`ALTER TABLE magic_item_equipped ADD COLUMN temper INTEGER NOT NULL DEFAULT 0`,
+		// Revisit R1 (gogobee_revisit_plan.md §R1). Until now "how far along
+		// is this run" and "which room am I standing in" were the same number,
+		// both read off len(visited_nodes). Backtracking splits them: the
+		// path index stops being monotonic, so effort gets its own counter.
+		// DEFAULT 0 is wrong for in-flight rows — bootstrapRoomsTraversed
+		// backfills them from visited_nodes.
+		`ALTER TABLE dnd_zone_run ADD COLUMN rooms_traversed INTEGER NOT NULL DEFAULT 0`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {