ux(showcase): pipeline fetches in-order, chunk size 3, keep the trickle

Operator-flagged 2026-05-30 (round 3): the all-parallel fetch was fast
but could let later chunks arrive ahead of earlier ones — even when
each chunk is a random sample, that "later chunk loads first" risk made
the load order non-deterministic. And the original goal behind asking
for batching was a faster first-image-on-screen, which neither sequen-
tial nor parallel really addressed cleanly.

Switched the loadInitial flow to a PIPELINE:
- Only one fetch in flight at any moment (in-order arrival, no race).
- The NEXT fetch kicks off as soon as the current one resolves (NOT
  after its trickle finishes), so the next RTT overlaps the visible
  trickle window — round-trips are hidden behind the animation cadence.
- PAGE 5 → 3 + INITIAL_BATCHES 12 → 20 (total still 60). Smaller chunk
  → first chunk's items appear sooner (a chunk of 3 trickles in 240ms,
  well within one RTT, so by the time chunk 2 is in-hand the first
  trickle is just finishing).

Trickle, sequence-token guard, and infinite-scroll behaviour unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/showcase.js

@@ -2,20 +2,22 @@ import { defineStore } from 'pinia'
 import { ref, computed } from 'vue'
 import { useApi } from '../composables/useApi.js'
 
-// Operator-flagged 2026-05-30 (round 2): the per-batch-then-pause cadence
-// from the original sequential-fetch implementation still felt "chunky" —
-// 5 items appear together, then the API round-trip pauses for ~200 ms,
-// then another 5 burst in. To smooth that:
-//   1. Fire all INITIAL_BATCHES fetches IN PARALLEL — collapses the per-
-//      batch round-trip gap (12 concurrent calls return in ~1 RTT instead
-//      of 12 sequential RTTs).
-//   2. Trickle responses into images.value one item at a time with a
-//      small APPEND_DELAY_MS delay between each — the user sees a steady
-//      cadence rather than per-batch bursts.
-// fetchPage (the infinite-scroll path) reuses the same trickle so its
-// 5-item appends also cascade one-by-one instead of popping in together.
-const PAGE = 5
-const INITIAL_BATCHES = 12
+// Operator-flagged 2026-05-30 (round 3): the all-parallel fetch was fast
+// but risked later chunks arriving first — undesirable even when each
+// chunk is a random sample. Switched to a PIPELINE: only one fetch in
+// flight at any moment, but the next fetch kicks off as soon as the
+// previous one resolves (NOT after its trickle finishes). The next RTT
+// overlaps with the current batch's trickle, hiding the per-batch
+// round-trip behind the visible animation cadence. Responses arrive in
+// fire-order, so no out-of-order rendering surprises.
+//
+// Smaller PAGE (3 vs 5) → first chunk's items appear sooner: a chunk of
+// 3 trickles in 240 ms, well within one RTT, so by the time chunk 2 is
+// in-hand the trickle is just finishing. Total wall-clock is roughly
+// RTT + N × max(trickle_time, RTT); APPEND_DELAY_MS keeps the visible
+// cadence smooth throughout.
+const PAGE = 3
+const INITIAL_BATCHES = 20
 const APPEND_DELAY_MS = 80  // ≈ the MasonryGrid stagger animation (70 ms)
 
 
@@ -64,10 +66,19 @@ export const useShowcaseStore = defineStore('showcase', () => {
     }
   }
 
-  // Reset state, fire INITIAL_BATCHES fetches in parallel, then trickle
-  // each response's items into images.value at APPEND_DELAY_MS cadence.
-  // Total wall-clock = ~1 RTT (parallel) + N items × APPEND_DELAY_MS;
-  // the user sees a smooth steady stream throughout.
+  function _fetchOne() {
+    return api.get('/api/showcase', { params: { limit: PAGE } }).catch(e => {
+      error.value = error.value || (e.message || String(e))
+      return null
+    })
+  }
+
+  // Reset state and pipeline INITIAL_BATCHES fetches: only one in flight
+  // at a time, but kick off the next one as soon as the previous resolves
+  // (NOT after its trickle finishes), so the next RTT runs concurrently
+  // with the current batch's trickle. Responses arrive in fire-order, so
+  // items always render in the order they were fetched — no out-of-order
+  // surprises from parallel races.
   async function loadInitial() {
     _seq += 1
     const mySeq = _seq
@@ -77,16 +88,17 @@ export const useShowcaseStore = defineStore('showcase', () => {
     error.value = null
     loading.value = true
     try {
-      const fetches = Array.from({ length: INITIAL_BATCHES }, () =>
-        api.get('/api/showcase', { params: { limit: PAGE } }).catch(e => {
-          error.value = error.value || (e.message || String(e))
-          return null
-        })
-      )
-      for (const fetchPromise of fetches) {
+      let nextFetch = _fetchOne()
+      for (let i = 0; i < INITIAL_BATCHES; i++) {
         if (mySeq !== _seq) return
-        const body = await fetchPromise
-        if (body && body.images) await _trickleAppend(body.images, mySeq)
+        const body = await nextFetch
+        // Fire the NEXT fetch immediately so its RTT overlaps the trickle.
+        if (i + 1 < INITIAL_BATCHES) nextFetch = _fetchOne()
+        if (!body || !body.images || body.images.length === 0) {
+          exhausted.value = true
+          break
+        }
+        await _trickleAppend(body.images, mySeq)
       }
       if (mySeq === _seq && images.value.length === 0) exhausted.value = true
     } finally {