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.

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