Add op_assert grounded-envelope primitive and purview-bounded mutation wrappers

Adds engram_assert_json — a grounded "assertion envelope" primitive for a
realizer/op_assert seam (per backlog bl-53/#57) — plus purview-scoped
mutation wrappers engram_node_full_in/engram_connect_in, which refuse
non-default purviews rather than silently mutating the live store. Threads
through el_seed.c/h wrappers and the codegen.el arity table per the
project's existing C-builtin recipe.

Also rewrites lang/AGENTS.md build docs with verified (2026-08-15) findings
that el_seed.c does not compile standalone.

lang/el-compiler/runtime/el_runtime.h

@@ -639,6 +639,14 @@ el_val_t  engram_scan_nodes_by_type_json(el_val_t node_type, el_val_t limit, el_
 el_val_t  engram_neighbors_json(el_val_t node_id, el_val_t max_depth, el_val_t direction);
 el_val_t  engram_activate_json(el_val_t query, el_val_t depth);
 el_val_t  engram_stats_json(void);
+/* op_assert seam: grounded assertion envelope {subject,grounding} for the realizer. */
+el_val_t  engram_assert_json(el_val_t node_id, el_val_t depth);
+/* Parametric mutation (purview write-side): purview==0 => G=live (default), else refuse. */
+el_val_t  engram_node_full_in(el_val_t purview, el_val_t content, el_val_t node_type, el_val_t label,
+                              el_val_t salience, el_val_t importance, el_val_t confidence,
+                              el_val_t tier, el_val_t tags);
+void      engram_connect_in(el_val_t purview, el_val_t from_id, el_val_t to_id,
+                            el_val_t weight, el_val_t relation);
 el_val_t  engram_list_layers_json(void);
 /* engram_compile_layered_json — produce a prompt-ready text block split
  * into "[LAYER 0 — STRUCTURAL]" (non-suppressible layers, sacred fire)