host: Playwright check for the relate picker (+ 2 bugs it caught)

Wire a browser check for the picker, run it against an ephemeral host server,
and fix the two real bugs it surfaced.

- lib/host/playwright/relate-picker.spec.js — drives login-redirect-return,
  JS candidate load + infinite scroll, debounced filter, and click-to-relate
  (asserting the relation shows on the post page).
- lib/host/playwright/run-picker-check.sh — spins up an ephemeral host server
  (this worktree's binary + lib, temp persist), seeds a host post + 25
  candidates, runs the spec in the main worktree's Playwright/chromium, tears
  everything down. No live-site dependency, no live-data pollution. 4/4 pass.

Bugs the check caught:
1. Query params weren't %-decoded — dream's form parser decodes but its query
   parser doesn't, so a filter "Item 13" arrived as "Item%2013" and matched
   nothing. Fix: decode q with dream's own dr/url-decode in host/blog-relate-
   options. (+ conformance test for a spaced filter.)
2. A filter typed while a load was in flight got dropped (busy guard returned
   with no trailing fetch). Fix: a `pending` flag re-runs the load when the
   in-flight one finishes, coalescing to the latest query.

239/239 conformance; JS node --check clean. Verified live: spaced filter
returns matches; served JS carries the pending-reload fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/tests/blog.sx

@@ -261,6 +261,10 @@
   (let ((body (dream-resp-body (host-bl-app (host-bl-req "/alpha-post/relate-options?q=beta")))))
     (list (contains? body "Beta Post") (contains? body "Gamma Post")))
   (list true false))
+(host-bl-test "relate-options filter url-decodes q (spaces)"
+  (let ((body (dream-resp-body (host-bl-app (host-bl-req "/alpha-post/relate-options?q=Beta%20Post")))))
+    (list (contains? body "Beta Post") (contains? body "Gamma Post")))
+  (list true false))
 (host-bl-test "relate-options excludes already-related candidates"
   (begin
     (host/blog-relate! "alpha-post" "beta-post")