test(vision): tighten spawn_blocking interval test against Burst-replay (0.87.22)

0.87.14 followup. The 0.87.14 test used MissedTickBehavior::Burst —
any post-prep yield replayed missed ticks and could fake the >= 2
threshold. Switch to Skip and assert on `ticks_post - ticks_pre >= 50`,
which sits well above the inlined-prep ceiling (~5) and far below
the spawn_blocking floor (~700+). Mutation-confirmed.

Also corrects: prepare_analysis doc count ("five fallback paths" → 4).

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.87.21",
+  "version": "0.87.22",
   "private": true,
   "type": "module",
   "scripts": {