H7: adjacency streams — per-(node,kind) edge reads, no more full kv scans (TDD)

Failing tests first (2 red: relate! wrote no adjacency streams). Every edge write now maintains
per-pair event streams (rel:src|kind ← {:dst}, rin:dst|kind ← {:src}); host/blog-out/-in/--out-raw
(+ new --in-raw) fold ONLY the pair's stream — O(edges of that node under that kind) instead of
O(all kv keys) per read. Append-only ⇒ no read-modify-write race (duplicate :adds fold to a set).
The edge:* kv rows remain (whole-graph consumers: subtype-closure, relations admin block) and feed
host/blog-reindex-edges! — the idempotent boot migration serve.sh now runs, so pre-H7 live stores
read correctly. Collapsed host/blog--add-edge-kv! into add-edge! (the type-algebra conj/disj edges
were bypassing the streams — caught by the existing algebra tests going red).

blog suite 256/256 (+6); FULL conformance 658/658.

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

lib/host/serve.sh

@@ -304,6 +304,10 @@ EPOCH=1
   echo "(eval \"(host/blog--set-actor! \\\"${SX_ACTOR:-site}\\\" \\\"${SX_SELF_URL:-}\\\")\")"
   EPOCH=$((EPOCH+1))
   echo "(epoch $EPOCH)"
+  # H7: migrate legacy edge:* rows into the per-pair adjacency streams (idempotent).
+  echo "(eval \"(host/blog-reindex-edges!)\")"
+  EPOCH=$((EPOCH+1))
+  echo "(epoch $EPOCH)"
   echo "(eval \"(host/blog-load-followers!)\")"
   EPOCH=$((EPOCH+1))
   echo "(epoch $EPOCH)"