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,