host: typed relations — Phase 1.5, durable edge store + boot replay

lib/relations holds the graph in memory only (a Datalog cache), so related/tags/
types were wiped on every restart while the posts (durable KV) survived — fatal
for a model where tags and types ARE relations. Make the host the durable source
of truth.

- every physical edge is also a KV row "edge:<src>|<kind>|<dst>" in the blog
  store (host/blog--add-edge!/--del-edge! wrap relations/relate+unrelate with
  kv-put/kv-delete). '|' is safe: slugs are [a-z0-9-], kinds are registry names.
- host/blog-load-edges! rebuilds the in-memory graph from edge:* keys; serve.sh
  calls it on boot right after pointing the store at the durable backend.
- lib/relations stays an in-memory cache; the durable KV is the source of truth
  (same shape as the blog pointing at the durable backend).

3 tests: KV row written on relate, replay rebuilds the graph after an in-memory
wipe (restart sim), unrelate deletes the row. 247/247.

Verified live: related welcome<->hello, force-recreated the container (wipes the
in-memory graph), the relation + its rendered block survived the restart.

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

lib/host/serve.sh

@@ -105,6 +105,12 @@ EPOCH=1
   echo "(epoch $EPOCH)"
   echo "(eval \"(host/blog-use-store! (persist/durable-backend))\")"
   EPOCH=$((EPOCH+1))
+  # Rebuild the relations graph from the durable edge store. lib/relations holds
+  # the graph in memory only, so without this, related/tags/types vanish on every
+  # restart even though the posts persist.
+  echo "(epoch $EPOCH)"
+  echo "(eval \"(host/blog-load-edges!)\")"
+  EPOCH=$((EPOCH+1))
   # Session signing secret + admin login credentials, then grant the admin
   # principal "edit" on "blog" so a logged-in session passes the ACL gate on the
   # write routes. Sessions stay IN-MEMORY (default store) — logins reset on