feat(explore): reach dial to escape dense clusters + anti-revisit (#1476)

The Explore walk got stuck in dense signatures — neighbours all too similar, so
forward-arrow couldn't escape and Random was the only exit. Root cause: MMR only
diversifies WITHIN the nearest ~400 pool; in a dense cluster that whole pool is
near-identical, so there's no escape route in it.

- gallery_service.similar(reach=0.0, exclude_ids=None): reach>0 widens the pool
  (cap 400→1000) and _reach_sample strides across an outward-growing distance span
  so the set handed to MMR spans near→mid-far (guaranteed escape routes), not just
  the tight cluster. exclude_ids drops already-walked images. Gallery 'more like
  this' (reach=0) is unchanged.
- api/gallery similar: parse reach + exclude_ids.
- explore store: default reach 0.4 (auto-diversifies without touching the dial),
  pass the breadcrumb as exclude_ids, setReach action.
- ExploreView: a Near↔Far reach slider in the trail.
- tests: _reach_sample math (deeper ranks with higher reach, near kept); similar
  exclude_ids drops walked + reach path runs clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/stores/explore.js

@@ -25,6 +25,10 @@ export const useExploreStore = defineStore('explore', () => {
   const cursor = ref(-1)
   const loading = ref(false)
   const error = ref(null)
+  // Reach (#1476): how far the walk reaches past the anchor's immediate cluster.
+  // 0 = nearest (can get stuck in a dense signature); ~0.4 default mixes in
+  // mid-far escape routes so the walk diversifies without hitting "Random image".
+  const reach = ref(0.4)
 
   const inflight = useInflightToken()
 
@@ -46,7 +50,13 @@ export const useExploreStore = defineStore('explore', () => {
         const body = await api.get('/api/gallery/similar', {
           // exclude_wip: keep work-in-progress out of the Explore rabbit-hole
           // (the gallery's own "similar" button still shows it) — operator 2026-07-08.
-          params: { similar_to: numId, limit: NEIGHBOR_LIMIT, exclude_wip: 1 },
+          // reach + exclude_ids (#1476): reach past the dense cluster + never re-serve
+          // an already-walked image, so the walk keeps moving instead of getting stuck.
+          params: {
+            similar_to: numId, limit: NEIGHBOR_LIMIT, exclude_wip: 1,
+            reach: reach.value,
+            exclude_ids: breadcrumb.value.map((c) => c.id).join(','),
+          },
         })
         if (!t.isCurrent()) return
         neighbors.value = body.images || []
@@ -113,6 +123,14 @@ export const useExploreStore = defineStore('explore', () => {
     loading.value = false
   }
 
+  // Change how far the walk reaches and re-fetch the current anchor's neighbours
+  // with the new setting (the anchor + trail are unchanged — only the grid varies).
+  function setReach (v) {
+    reach.value = Math.max(0, Math.min(1, Number(v)))
+    const id = anchor.value?.id
+    if (id != null) anchorOn(id)
+  }
+
   // --- TagPanel "host" surface ---------------------------------------------
   // The anchor IS the current image (same /api/gallery/image/<id> payload the
   // modal uses), so these mirror the modal store's tag-CRUD, targeting the
@@ -189,8 +207,8 @@ export const useExploreStore = defineStore('explore', () => {
   function close () {}
 
   return {
-    anchor, neighbors, breadcrumb, cursor, loading, error, NEIGHBOR_LIMIT,
-    anchorOn, reset, backTarget, forwardTarget,
+    anchor, neighbors, breadcrumb, cursor, loading, error, NEIGHBOR_LIMIT, reach,
+    anchorOn, reset, backTarget, forwardTarget, setReach,
     // host surface
     current, currentImageId,
     reloadTags, addExistingTag, removeTag, createAndAdd, close,

frontend/src/views/ExploreView.vue

@@ -31,6 +31,20 @@
           <img :src="c.thumbnail_url" alt="" loading="lazy" />
         </button>
         <div class="fc-ex__trail-actions">
+          <!-- Reach (#1476): how far each step reaches past the anchor's cluster.
+               Raise it to break out of a dense signature without hitting Random. -->
+          <div
+            class="fc-ex__reach"
+            title="How far each step reaches — raise it to escape a dense cluster without going fully random"
+          >
+            <v-icon size="16" color="accent">mdi-map-marker-distance</v-icon>
+            <v-slider
+              :model-value="store.reach" @end="store.setReach"
+              :min="0" :max="1" :step="0.2" hide-details density="compact"
+              color="accent" class="fc-ex__reach-slider"
+            />
+            <span class="fc-muted fc-ex__reach-label">{{ reachLabel }}</span>
+          </div>
           <!-- Active retrain right where you tag: fold the +/- you just gave
                into the heads without a trip to Settings (the nightly beat is the
                passive cadence). -->
@@ -153,6 +167,12 @@ const modal = useModalStore()
 
 const anchorId = computed(() => route.params.imageId || null)
 const isVideo = computed(() => !!store.anchor?.mime?.startsWith('video/'))
+const reachLabel = computed(() => {
+  const r = store.reach
+  if (r <= 0.15) return 'Near'
+  if (r <= 0.55) return 'Varied'
+  return 'Far'
+})
 
 // #1206: hovering a suggestion in the rail highlights the crop it came from on
 // the anchor image (same provide/inject as the modal viewer).
@@ -296,6 +316,14 @@ onUnmounted(() => {
 .fc-ex__trail-actions {
   margin-left: auto; display: flex; align-items: center; gap: 4px; flex: 0 0 auto;
 }
+.fc-ex__reach {
+  display: flex; align-items: center; gap: 6px;
+  margin-right: 8px;
+}
+.fc-ex__reach-slider { width: 96px; }
+.fc-ex__reach-label {
+  font-size: 12px; min-width: 44px; text-align: left;
+}
 
 /* The three panes fill the remaining height; each scrolls on its own.
    grid-template-rows: minmax(0, 1fr) BOUNDS the single row to the container