host P2: all state changes emit canonical activities (LIVE-VERIFIED)

Generalizes emission beyond publish to the full event source. TWO ActivityPub-faithful classes:
- CONTENT (host/blog--content-activity): Create on first publish, Update on a subsequent published
  edit. object-type is DERIVED from the post's is-a (host/blog--post-type), not hardcoded 'article'.
- RELATION (host/blog--relation-activity): Add/Remove, carrying :relation + :target (the edge).

host/blog--emit! runs any activity through behavior/process (logged + matched). emit-content-change!
(create/update) wired into form-submit + edit-submit; emit-relation! (add/remove) wired into
relate-submit + unrelate-submit.

DEBT #1 FIXED — per-EVENT :id (not the bare CID): content = create:/update:+cid; relation =
add:/remove:+src:kind:dst (EDGE-based, because a relation change doesn't shift the CID, so a
CID-based id would false-dedup different edges on one object).

The activity log is now the DURABLE EVENT SOURCE (string-keyed records under 'activitylog',
boot-loaded), surfaced at /activities — what TA will push to peers.

LIVE PROOF (blog.rose-ash.com): publish → /activities 'create article <cid>'; relate → 'add article
p2-events — add welcome related'; unrelate → 'remove …'. blog 217/217 (+4 P2, reframed P0.3 fire
tests for Update semantics), full host conformance 614/614.

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

lib/host/serve.sh

@@ -155,6 +155,10 @@ EPOCH=1
   echo "(epoch $EPOCH)"
   echo "(eval \"(host/blog-load-flowlog!)\")"
   EPOCH=$((EPOCH+1))
+  # P2: rebuild the activity log (the event source) from the durable store, so /activities survives.
+  echo "(epoch $EPOCH)"
+  echo "(eval \"(host/blog-load-activitylog!)\")"
+  EPOCH=$((EPOCH+1))
   # Sessions on the DURABLE store, LAZILY: only a logged-in session (one that
   # writes a field) persists, so a login survives a restart while anonymous /
   # crawler traffic leaves no rows. host/session-init! bumps the per-boot epoch