hover black space around mosaic: per-free-dir dropzones + hotspots

Mouse users can now hover the empty black space beside the current tile to
preview the photo that would be placed at that free-direction spot.

renderTileZones now renders one dropzone per free direction (N/E/S/W):
- the most-centered free dir keeps the persistent .preview (thumbnail)
- the other free dirs are transparent hover hotspots that showGhost() the
  destination placement on pointermove and scheduleHideGhost() on leave
- stable-element logic via a key string: early-return when unchanged; if
  only bestDir flips, move the cached prevEl into the new best dropzone and
  update img.src (no element churn / no flicker)
- hotspots skipped on TOUCH (kept for tap-to-walk), persistent preview only
- one preview at a time via .dropzone.hidden { visibility:hidden }

zones map entries are now { els, key, bestDir, prevEl, img }.

Tests green: domtest 58, touchtest 19, exhausttest 9, splashtest 6, fuzz clean,
hoverlive 0 stray ghosts. Live at ?v=27.

1 files changed, 1 insertions, 1 deletions

diff --git a/public/index.html b/public/index.html
index 1514600..549ca5d 100644
--- a/public/index.html
+++ b/public/index.html
@@ -16,6 +16,6 @@
     <p>four paths, one per direction. walk them.</p>
     <button id="start">start</button>
   </div>
-  <script src="/app.js?v=26"></script>
+  <script src="/app.js?v=27"></script>
 </body>
 </html>