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.

2 files changed, 48 insertions, 31 deletions

diff --git a/public/app.js b/public/app.js
index cce9bc8..11f19b9 100644
--- a/public/app.js
+++ b/public/app.js
@@ -348,8 +348,8 @@ let ghostVisible = false
 function setPreviewHidden(hidden) {
   ghostVisible = hidden
   if (!cur) return
-  const zs = zones.get(cur.id)
-  if (zs) for (const z of zs) z.classList.toggle("hidden", hidden)
+  const entry = zones.get(cur.id)
+  if (entry) for (const z of entry.els) z.classList.toggle("hidden", hidden)
 }
 function showGhost(p) {
   if (!p) return
@@ -392,9 +392,9 @@ function cancelHideGhost() {
 
 const zones = new Map()
 function clearTileZones(id) {
-  const zs = zones.get(id)
-  if (zs) {
-    for (const z of zs) world.removeChild(z)
+  const entry = zones.get(id)
+  if (entry) {
+    for (const z of entry.els) world.removeChild(z)
     zones.delete(id)
   }
 }
@@ -408,42 +408,59 @@ function renderTileZones(id) {
   if (!tiles.has(id) || !exitsCache.has(id)) return
   const cx = (window.innerWidth / 2 - panX) / scale
   const cy = (window.innerHeight / 2 - panY) / scale
-  let bestDir = null
-  let best = null
-  let bestD = Infinity
+  const dirs = []
   for (const dir of ["N", "E", "S", "W"]) {
     if (adjacentTile(id, dir)) continue
     const p = choosePlacement(id, dir)
     if (!p) continue
     const d = Math.hypot(p.rect.x + p.rect.w / 2 - cx, p.rect.y + p.rect.h / 2 - cy)
-    if (d < bestD) { bestD = d; bestDir = dir; best = p }
+    dirs.push({ dir, p, d })
+  }
+  const key = dirs.map((x) => x.dir + x.p.info.id + "@" + x.p.rect.x + "," + x.p.rect.y).join("|")
+  let best = null
+  for (const x of dirs) if (!best || x.d < best.d) best = x
+  const entry = zones.get(id)
+  if (entry && entry.key === key) {
+    if (best && best.dir !== entry.bestDir) {
+      const z = entry.els.find((el) => el.dataset.dir === best.dir)
+      if (z) {
+        z.appendChild(entry.prevEl)
+        entry.img.src = best.p.info.thumb
+        entry.bestDir = best.dir
+      }
+    }
+    return
   }
-  const existing = zones.get(id) && zones.get(id)[0]
-  if (best && existing &&
-      existing.dataset.dir === bestDir &&
-      existing.dataset.key === best.info.id + "|" + best.rect.x + "," + best.rect.y) return
   clearTileZones(id)
   if (!best) return
-  const z = document.createElement("div")
-  z.className = "dropzone"
-  z.dataset.dir = bestDir
-  z.dataset.key = best.info.id + "|" + best.rect.x + "," + best.rect.y
-  z.style.left = best.rect.x + "px"
-  z.style.top = best.rect.y + "px"
-  z.style.width = best.rect.w + "px"
-  z.style.height = best.rect.h + "px"
-  const prev = document.createElement("div")
-  prev.className = "preview"
+  const els = []
+  const prevEl = document.createElement("div")
+  prevEl.className = "preview"
   const img = document.createElement("img")
-  img.src = best.info.thumb
+  img.src = best.p.info.thumb
   img.alt = ""
   img.loading = "lazy"
-  prev.appendChild(img)
-  z.appendChild(prev)
-  z.addEventListener("pointerdown", (e) => e.stopPropagation())
-  z.addEventListener("click", () => traverseFrom(id, bestDir))
-  world.appendChild(z)
-  zones.set(id, [z])
+  prevEl.appendChild(img)
+  for (const x of dirs) {
+    const z = document.createElement("div")
+    z.className = "dropzone"
+    z.dataset.dir = x.dir
+    z.style.left = x.p.rect.x + "px"
+    z.style.top = x.p.rect.y + "px"
+    z.style.width = x.p.rect.w + "px"
+    z.style.height = x.p.rect.h + "px"
+    z.addEventListener("pointerdown", (e) => e.stopPropagation())
+    z.addEventListener("click", () => traverseFrom(id, x.dir))
+    if (!TOUCH && x.dir !== best.dir) {
+      z.addEventListener("pointermove", () => { cancelHideGhost(); showGhost(x.p) })
+      z.addEventListener("pointerleave", () => scheduleHideGhost())
+    }
+    world.appendChild(z)
+    els.push(z)
+  }
+  const bestEl = els.find((el) => el.dataset.dir === best.dir)
+  bestEl.appendChild(prevEl)
+  zones.set(id, { els, key, bestDir: best.dir, prevEl, img })
   setPreviewHidden(ghostVisible)
 }
 
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>