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_seed.c

@@ -1095,6 +1095,15 @@ el_val_t __engram_activate_json(el_val_t query, el_val_t depth) {
 }
 
 el_val_t __engram_stats_json(void)                         { return engram_stats_json(); }
+el_val_t __engram_assert_json(el_val_t node_id, el_val_t depth) { return engram_assert_json(node_id, depth); }
+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) {
+    return engram_node_full_in(purview, content, node_type, label, salience, importance, confidence, tier, 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) {
+    engram_connect_in(purview, from_id, to_id, weight, relation);
+}
 el_val_t __engram_list_layers_json(void)                   { return engram_list_layers_json(); }
 
 el_val_t __engram_compile_layered_json(el_val_t intent, el_val_t depth) {