engram tiered storage: engram-service wiring + elc fold-hang fix + prune-store mirror

- Wire paged store into the ENGRAM SERVICE (server.el, the authoritative durable
  owner): boot->engram_store_boot, persist_canonical->engram_store_checkpoint,
  gated by ENGRAM_STORE.
- elc (lang/elc.c + src/parser.el + codegen.el + elc-combined.el): OOB guard in
  tok_kind/tok_value + parse_block progress backstop — fixes the pre-existing
  unbounded-memory fold hang on sessions.el.
- engram_prune_telemetry mirrors ISE prune to the store (store_forget) so store
  live-count tracks resident and stale telemetry stays bounded.
- Deployed live 2026-08-12: engram :8742 on neuron.egm+WAL, count reconciled 11552.

lang/elc-combined.el

@@ -3797,6 +3797,9 @@ fn builtin_arity(name: String) -> Int {
     if str_eq(name, "engram_activate") { return 2 }
     if str_eq(name, "engram_save") { return 1 }
     if str_eq(name, "engram_load") { return 1 }
+    if str_eq(name, "engram_store_boot") { return 1 }
+    if str_eq(name, "engram_store_checkpoint") { return 0 }
+    if str_eq(name, "engram_store_close") { return 0 }
     if str_eq(name, "engram_get_node_json") { return 1 }
     if str_eq(name, "engram_search_json") { return 2 }
     if str_eq(name, "engram_scan_nodes_json") { return 2 }