Previews appear as you scroll: current tile follows pan, tappable destination previews

- Current tile follows the viewport while dragging (touch) or scrolling
  (wheel), throttled ~200ms, snapping to the nearest visible tile on
  gesture end. setCurrent(id, rect, recenter) so follow never fights the
  user's pan (no recenter, no zoom change).
- Persistent destination previews: the current tile's free exits now
  render the destination photo (ghost) at each spot, always visible on
  mobile and desktop, replacing the old all-tile invisible dropzones and
  the hover-only dropzone ghost. Tapping/clicking a preview walks there.
- Desktop hover ghost kept for non-current tiles only (the current tile's
  previews are already persistent, so no duplicate).
- Dropzones/previews are current-tile-only; ghost preview on nav zones of
  the current tile suppressed.
- Removed MAX_ZONE_TILES (unused after current-only previews).

Tests: domtest 49/49 (rewritten ghost checks for persistent previews +
follow-scroll), touchtest 19/19, exhaust 9/9, splash 6/6, fuzz clean,
hoverlive against live data: 0 stray ghosts, 31 previews across 13 tiles.

1 files changed, 2 insertions, 2 deletions

diff --git a/public/index.html b/public/index.html
index 0beff50..1a30ce1 100644
--- a/public/index.html
+++ b/public/index.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <meta name="color-scheme" content="dark">
   <title>pathways</title>
-  <link rel="stylesheet" href="/app.css?v=21">
+  <link rel="stylesheet" href="/app.css?v=22">
 </head>
 <body>
   <main id="stage">
@@ -16,6 +16,6 @@
     <p>four paths, one per direction. walk them.</p>
     <button id="start">start</button>
   </div>
-  <script src="/app.js?v=21"></script>
+  <script src="/app.js?v=22"></script>
 </body>
 </html>