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.c

@@ -8410,6 +8410,59 @@ el_val_t engram_activate_json(el_val_t query, el_val_t depth) {
     return el_wrap_str(jb_finish(&b));
 }
 
+/* op_assert seam (realizer promotion, bl-53/#57).
+ * Gathers the grounded ASSERTION ENVELOPE for a subject node —
+ *   { "subject": <node|null>, "grounding": [ {node,edge,hops}... ] }
+ * i.e. the self-geometry a realizer renders as faithful first-person text.
+ * Read-only: realization (geometry->text) stays in the faculty/realizer;
+ * this native primitive produces its structured input from proven paths
+ * (engram_emit_node_json + engram_neighbors_json). arity 2 (node_id, depth). */
+el_val_t engram_assert_json(el_val_t node_id, el_val_t depth) {
+    const char* sid = EL_CSTR(node_id);
+    JsonBuf b; jb_init(&b);
+    jb_puts(&b, "{\"subject\":");
+    EngramNode* n = (sid && *sid) ? engram_find_node(sid) : NULL;
+    if (n) engram_emit_node_json(&b, n); else jb_puts(&b, "null");
+    jb_puts(&b, ",\"grounding\":");
+    el_val_t nb = engram_neighbors_json(node_id, depth, EL_STR("both"));
+    const char* nbs = EL_CSTR(nb);
+    jb_puts(&b, (nbs && *nbs) ? nbs : "[]");
+    jb_putc(&b, '}');
+    return el_wrap_str(jb_finish(&b));
+}
+
+/* Parametric mutation (purview write-side bounding, keystone 56ecbec6).
+ * The mutation verbs travel with a TARGET MANIFOLD (purview) instead of the
+ * implicit global singleton. purview==0 (EL_NULL) is the DEGENERATE/DEFAULT
+ * case: G = live, behaviour identical to the base op. A non-zero purview is a
+ * bounded target that the engine cannot yet resolve (multi-manifold store is a
+ * promotion item), so we REFUSE rather than silently mutate the live set —
+ * write-side bounding must never leak into G=live. */
+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) {
+    if (purview == 0) {
+        return engram_node_full(content, node_type, label, salience, importance,
+                                confidence, tier, tags);
+    }
+    fprintf(stderr, "[engram] purview write-side not yet resolvable (G != live); "
+                    "refusing to append to live store (purview=%lld)\n",
+            (long long)purview);
+    return EL_STR("");
+}
+
+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) {
+    if (purview == 0) {
+        engram_connect(from_id, to_id, weight, relation);
+        return;
+    }
+    fprintf(stderr, "[engram] purview write-side not yet resolvable (G != live); "
+                    "refusing to connect in live store (purview=%lld)\n",
+            (long long)purview);
+}
+
 el_val_t engram_stats_json(void) {
     EngramStore* g = engram_get();
     char buf[128];