feat(engram): ML-KEM-1024 PQC encryption, nomic embeddings, MCP routes, checkpoint-per-write

- Add ML-KEM-1024 + AES-256-GCM binary persistence to el_runtime.c with
  two-key scheme (Neuron master + user key); SHAKE-256 key derivation
- Add nomic-embed-text 768-dim float32 embeddings on every node write
  via Ollama; graceful fallback when Ollama is not running
- Wire all /api/neuron/* MCP routes directly into Engram (server.el),
  eliminating the Kotlin server as the MCP backend
- Set ENGRAM_CHECKPOINT_INTERVAL = 1 (write binary on every node write,
  not every 50)
- Add el_runtime.h declarations for engram_write_binary_el and
  engram_load_binary_el builtins

lang/dist/platform/elc.c

@@ -4546,6 +4546,12 @@ el_val_t builtin_arity(el_val_t name) {
   if (str_eq(name, EL_STR("engram_load"))) {
     return 1;
   }
+  if (str_eq(name, EL_STR("engram_load_dir"))) {
+    return 1;
+  }
+  if (str_eq(name, EL_STR("engram_reindex_json"))) {
+    return 0;
+  }
   if (str_eq(name, EL_STR("engram_get_node_json"))) {
     return 1;
   }
@@ -4564,6 +4570,9 @@ el_val_t builtin_arity(el_val_t name) {
   if (str_eq(name, EL_STR("engram_stats_json"))) {
     return 0;
   }
+  if (str_eq(name, EL_STR("engram_apply_decay_json"))) {
+    return 0;
+  }
   if (str_eq(name, EL_STR("llm_call"))) {
     return 2;
   }