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/runtime/el_runtime.h

@@ -605,6 +605,13 @@ el_val_t  engram_edge_count(void);
 el_val_t  engram_activate(el_val_t query, el_val_t depth);
 el_val_t  engram_save(el_val_t path);
 el_val_t  engram_load(el_val_t path);
+/* Tiered paged-store entry points (ENGRAM_STORE=1). engram_store_boot opens the
+ * durable store (import-once / WAL-replay) and loads it resident; checkpoint pushes
+ * the resident graph's current field state (incl. learned hebb + activation-formed
+ * edges) through the WAL and flushes; close checkpoints + closes. No-ops when off. */
+el_val_t  engram_store_boot(el_val_t data_dir);
+el_val_t  engram_store_checkpoint(void);
+el_val_t  engram_store_close(void);
 
 /* JSON-string accessors — return pre-serialized JSON so HTTP handlers
  * can pass results straight through without round-tripping ElList/ElMap