host: relations-as-posts slice 2 — relation metadata lives on relation-posts

is-a/subtype-of/tagged/related are now POSTS (each is-a a new `relation` root),
owning their metadata in a :rel slot {:symmetric :label :inverse-label}. The static
host/blog-rel-kinds registry is gone: kind-spec/rel-kinds/kind-symmetric? read the
relation-posts (via an in-memory cache), and the relation list derives from
host/blog-in "relation" "is-a".

Perform-budget fixes (a durable read inside the http-listen render VM raises
VmSuspended; too many per request 500s the page):
 - relation metadata is loaded into a cache at boot (host/blog-load-rel-kinds!,
   like load-edges!), so kind-spec is pure on render paths;
 - the initial edit page renders its pickers EMPTY (the load trigger fills each) —
   only the relate/unrelate FRAGMENT server-renders candidates (with-cands flag).
   Previously every edit page render did candidate-get × 4 pickers and 500'd.

host conformance 287/287 (+4 slice-2: kind-spec reads :rel, kind-symmetric? off the
post, unknown kind has no spec, rel-kinds derived from the graph). run-picker-check
3/3 (edit page boots, relate/unrelate flow works, no client errors).

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

lib/host/serve.sh

@@ -151,6 +151,11 @@ EPOCH=1
   echo "(epoch $EPOCH)"
   echo "(eval \"(host/blog-seed-types!)\")"
   EPOCH=$((EPOCH+1))
+  # Load relation metadata (symmetry/labels) from the relation-posts into the
+  # in-memory cache, so render paths read it without a (VmSuspending) durable read.
+  echo "(epoch $EPOCH)"
+  echo "(eval \"(host/blog-load-rel-kinds!)\")"
+  EPOCH=$((EPOCH+1))
   # Index the web-stack .sxbc by content hash so /sx/h/{hash} can serve them
   # immutably and the shell can emit the data-sx-manifest (content-addressed
   # client module cache). Done once at boot.