host: Phase 3 — relations READ cut-over (get-children/get-parents), 121/121

Migrate the two internal relations read queries onto lib/relations: GET
/internal/data/get-children + /get-parents dispatch to relations/children
and relations/parents. Bridge the Quart (type,id) node key to a graph atom
symbol "type:id" with relation-type as the edge kind; optional child/parent
-type params filter by "type:" prefix. Golden tests pin each endpoint to
subsystem-call + envelope. Ledger entries flipped to :migrated (off-Quart
coverage 27% -> 45%).

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

lib/host/ledger.sx

@@ -18,16 +18,16 @@
 
 ;; ── the catalogue ───────────────────────────────────────────────────
 ;; Reflects the live host: feed reads+writes migrated, /health native, the
-;; internal-only likes/relations data+action endpoints still proxied to Quart.
-;; relations is the next cut-over candidate — it already has a real SX subsystem
-;; (lib/relations); likes has none, so it stays proxied until one exists.
+;; relations READ endpoints migrated onto lib/relations (see lib/host/relations.sx),
+;; relations writes + the internal-only likes data+action endpoints still proxied
+;; to Quart. likes has no SX subsystem, so it stays proxied until one exists.
 (define host/ledger
   (list
     (host/ledger-entry "host"      "GET"  "/health"                     nil                         "native"   "host/health-route")
     (host/ledger-entry "feed"      "GET"  "/feed"                       "feed:timeline"             "migrated" "host/feed-timeline")
     (host/ledger-entry "feed"      "POST" "/feed"                       "feed:create"               "migrated" "host/feed-create")
-    (host/ledger-entry "relations" "GET"  "/internal/data/get-children" "relations:get_children"    "proxied"  nil)
-    (host/ledger-entry "relations" "GET"  "/internal/data/get-parents"  "relations:get_parents"     "proxied"  nil)
+    (host/ledger-entry "relations" "GET"  "/internal/data/get-children" "relations:get_children"    "migrated" "host/relations-children")
+    (host/ledger-entry "relations" "GET"  "/internal/data/get-parents"  "relations:get_parents"     "migrated" "host/relations-parents")
     (host/ledger-entry "relations" "POST" "/internal/actions/relate"    "relations:relate"          "proxied"  nil)
     (host/ledger-entry "relations" "POST" "/internal/actions/unrelate"  "relations:unrelate"        "proxied"  nil)
     (host/ledger-entry "likes"     "GET"  "/internal/data/is-liked"     "likes:is_liked"            "proxied"  nil)