fix(explore): variance + no loop-back on → navigation (#94)

Two reports: → sometimes "loops back", and the walk gets stuck on near-identical
images. Cause: forwardTarget picked a uniformly-random neighbour from the 24
NEAREST, so it (a) often landed on an image already in the trail — which snaps
the cursor back into history and makes → bounce between visited nodes — and (b)
only ever offered near-duplicates.

forwardTarget now: excludes already-visited neighbours (→ opens something new,
no snap-back), and skips the closest third of the (similarity-sorted) pool so the
jump favours the more-varied remainder instead of lookalikes. Neighbour pool
widened 24→40 for more variety to browse + jump into. The post-← browser-forward
walk through visited crumbs is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/stores/explore.js

@@ -11,7 +11,7 @@ import { toast } from '../utils/toast.js'
 // trail. The store ALSO acts as a TagPanel "host" (current/currentImageId +
 // tag CRUD over the anchor) so the Explore workspace reuses the modal's tag
 // rail verbatim for modal-parity tagging while rabbit-holing.
-const NEIGHBOR_LIMIT = 24
+const NEIGHBOR_LIMIT = 40   // a wider pool → more variety to browse + jump into
 
 export const useExploreStore = defineStore('explore', () => {
   const api = useApi()
@@ -81,16 +81,26 @@ export const useExploreStore = defineStore('explore', () => {
     return cursor.value > 0 ? breadcrumb.value[cursor.value - 1].id : null
   }
 
-  // → target: the next already-visited crumb if we'd stepped back, else a
-  // RANDOM neighbour to keep the rabbit-hole going. Null if neither exists.
+  // → target: after a ←, walk forward through the already-visited trail
+  // (browser-style). Otherwise jump to a varied neighbour to keep the
+  // rabbit-hole going — null if neither exists.
   function forwardTarget () {
     if (cursor.value >= 0 && cursor.value < breadcrumb.value.length - 1) {
       return breadcrumb.value[cursor.value + 1].id
     }
-    if (neighbors.value.length) {
-      return neighbors.value[Math.floor(Math.random() * neighbors.value.length)].id
-    }
-    return null
+    if (!neighbors.value.length) return null
+    // Prefer UNVISITED neighbours so → opens something new instead of landing on
+    // a crumb (which snaps the cursor back into the trail — the "loops back"
+    // report). Fall back to the full set only if every neighbour's been seen.
+    const seen = new Set(breadcrumb.value.map((c) => c.id))
+    let pool = neighbors.value.filter((n) => !seen.has(n.id))
+    if (!pool.length) pool = neighbors.value
+    // neighbors come similarity-sorted (nearest first). Skip the closest slice —
+    // those near-duplicates are exactly what you get stuck cycling through — and
+    // pick from the more-varied remainder, for real variance in the walk.
+    const skip = pool.length >= 6 ? Math.floor(pool.length / 3) : 0
+    const cands = pool.slice(skip)
+    return cands[Math.floor(Math.random() * cands.length)].id
   }
 
   function reset () {